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A Jupiter-class Planet Orbiting a White Dwarf

A gas giant similar to Jupiter, and with a somewhat similar orbit, revolves around a white dwarf located about 6500 light years out toward galactic center. As reported in a paper in Nature, this is an interesting finding because stars like the Sun eventually wind up as white dwarfs, so we have to wonder what kind of planets could survive a star’s red giant phase and continue to orbit the primary. If Earth one day is engulfed, will the gas giants survive? The new discovery implies that result, and marks the first confirmed planetary system that looks like what ours could become.

Image: An artist’s rendition of a newly discovered Jupiter-like exoplanet orbiting a white dwarf. This system is evidence that planets can survive their host star’s explosive red giant phase, and is the first confirmed planetary system that serves as an analogue to the face of the Sun and Jupiter in our own Solar System. Credit: W. M. Keck Observatory/Adam Makarenko.

Underlining just how faint white dwarfs are is the method of discovery and the follow-up observations that made the paper on this work possible. A gravitational microlensing event called MOA-2010-BLG-477 was detected at Mount John Observatory (New Zealand) in 2010, later observed by more than 20 telescopes. A team led by Joshua Blackman (University of Tasmania) made infrared observations using the Keck Observatory’s adaptive optics system and its Near-Infrared Camera (NIRC2).

The microlensing analysis had revealed the star and its planet, while the Keck observations confirmed the faintness of the star. The paper’s analysis of the data is lengthy as the authors worked to rule out a variety of stellar possibilities in the main sequence given the faintness of the event. This is what emerged:

As all of the possible main-sequence lenses for the event are brighter than the Keck detection limit and no such star is observed, the lens cannot be a main-sequence star. The same analysis also excludes brown dwarf lenses owing to an upper limit on the microlensing parallax parameter, πE < 1.03, which leads to an implied limit on the lens system mass of ML > 0.15 M. Similarly, the lower microlensing parallax limit of πE > 0.26 implies an upper mass limit of ML < 0.78 M, which rules out neutron stars and black holes as the host stars. As main-sequence stars, brown dwarfs, neutron stars and black holes are ruled out, we conclude that the lens must be a white dwarf.

Image: This is Figure 1 from the paper. Caption: a, An image obtained with the narrow-camera on the NIRC2 imager in 2015 centred on MOA-2010-BLG-477 with an FOV of 8 arcsec. b, A 0.36-arcsec zoomed-in view of the same image as in a. The bright object in the centre is the source. To the northeast (top left) is an unrelated H = 18.52 ± 0.05 star 123 mas from the source, which we refer to as star 123NE. c, The field in 2018. The contours indicate the probable positions of a possible main-sequence host (probability of 0.393, 0.865, 0.989 from light to dark blue) using constraints from microlensing parallax and lens–source relative proper motion. No such host is detected. Credit: Blackman et al.

The authors used a sample of 130 white dwarfs within 20 parsecs of the Sun, excluding binary systems, and ran their calculations under the assumption that all white dwarfs are equally likely to host planets. We wind up with a white dwarf that is, typical of the type, about the size of the Earth, and about 55 percent the mass of the Sun. The gas giant is found to be approximately 40 percent more massive than Jupiter, orbiting at least 3 AU from the host. Thus we find our first analogue to the final stages of our own system some 2 kiloparsecs away toward the center of the galaxy.

It’s likely, according to this work, that the planet is indeed a survivor of the red giant phase of its host star, which in itself is an interesting aspect of the story. The authors discuss orbital change only sparingly, but point out that mass loss in the star pushes a planet toward a wider orbit, while tidal forces have the opposite effect when the star expands beyond about 1 AU. What little work I can find in the literature on this suggests a consensus that Jupiter-class planets orbiting white dwarfs are likely to be found at separations greater than 5 AU, higher than the ~3 AU we find here.

The paper is Blackman et al., “A Jovian analogue orbiting a white dwarf star,” Nature 598 (13 October 2021), 272-275 (abstract).



The 36 dish antennae at ASKAP — the Australian Square Kilometre Array Pathfinder in outback Western Australia — comprise an interferometer with a total collecting area of about 4,000 square meters. ASKAP has commanded attention as a technology demonstrator for the planned Square Kilometer Array, but today we’re looking at the discovery of a highly polarized, highly variable radio source labeled ASKAP J173608.2−321635, about 4 degrees from galactic center in the galactic plane.

According to Ziteng Wang, who is lead author of the study on this signal and a University of Sydney PhD student, the observations are strikingly different from other variable radio sources:

“The strangest property of this new signal is that it has a very high polarisation. This means its light oscillates in only one direction, but that direction rotates with time. The brightness of the object also varies dramatically, by a factor of 100, and the signal switches on and off apparently at random. We’ve never seen anything like it.”

Variable celestial objects are common enough, from supernovae to pulsars, not to mention interesting sources like Fast Radio Bursts and, of course, the Cepheid variable stars that have played such a large role in astronomical history in helping us determine the scale of the universe. Any new variable source might be looked upon in light of such objects, perhaps as a type of flare star intermittently spewing out bursts of radiation. But none of these match the odd behavior of the new source. While J173608.2−321635 was found at ASKAP, Wang and team performed follow-up observations with the MeerKET telescope in South Africa.

So we have a source toward galactic center that is at first unseen, then brightens, fades, and reappears. Having detected six such signals from the source over nine months in 2020, the astronomers searched in vain for it in visible light, even as a search with the Parkes radio telescope turned up nothing. That’s when the team turned to MeerKAT, where it was once again detected. Tara Murphy, who is Wang’s PhD supervisor at Sydney, notes what happened next:

“Because the signal was intermittent, we observed it for 15 minutes every few weeks, hoping that we would see it again. Luckily, the signal returned, but we found that the behaviour of the source was dramatically different — the source disappeared in a single day, even though it had lasted for weeks in our previous ASKAP observations.”

Image: The ASKAP telescope array. Credit: CSIRO.

Other low frequency transients from galactic center have been detected in recent years, including GCRT J1745-3009, which was quickly labeled a ‘burper’ by its discoverers due to its intermittent bursts after detection in 1998. Five bursts of equal brightness were noted, each about ten minutes in duration, and occurring every 77 minutes. No explanation has been agreed upon for that one either, although a pulsar, a neutron star pair, or a radio-emitting white dwarf have all been discussed in the literature.

For the ASKAP transient, the authors have considered pulsar scenarios, a transient magnetar, and “a low-mass star/substellar object with extremely low infrared luminosity,” with none of these providing a satisfactory answer. The suspicion grows that this is a new class of objects that future radio imaging surveys will observe as our capabilities improve. With the Square Kilometer Array coming online in the next decade, we are probably looking at a phenomenon that will generate a great deal of study and, doubtless, many more examples.

The paper is Wang et al., “Discovery of ASKAP J173608.2–321635 as a Highly Polarized Transient Point Source with the Australian SKA Pathfinder,” The Astrophysical Journal Vol. 920, No. 1 (12 October 2021), 45. Abstract.


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Enlarging Perspectives on Space (and Time)

What do we mean by an ‘interstellar mission’? The question came up in relation to Interstellar Probe, that ‘Voyager Plus’ concept being investigated by the Johns Hopkins Applied Physics Laboratory. I do indeed see it as an interstellar mission, as Interstellar Probe takes us outside the heliosphere and into the local interstellar medium. We need to understand conditions there because it would be folly to mount a mission to another star without knowing the dynamics of the heliosphere’s movement through the interstellar cloud we are currently in, or the ramifications of moving between it and the adjacent cloud as we make our crossing.

How could it be otherwise? Journeys need maps and knowledge of conditions along the way. Thus we push into the fringes of interstellar space, and gradually extend our reach. As we do this, we inevitably produce changes in the way we perceive our place in the cosmos.

Cultural expectations about space have been shaped by what I might call a ‘planar’ approach to astronomy. First there is the Moon, then Mars, then the main asteroid belt, and so on, all of these things at increasing distances but roughly along the great disk of the ecliptic. In the 1950s science fiction film Rocketship X-M, a Moon mission misses its target through a series of odd misadventures and winds up landing on Mars. It was entertaining in its way as Lloyd Bridges and team explored the Red Planet, but it depicts a view of the Solar System in which if you go one distance, you’re at one target, and if you go another, you’re at the next. Never mind that the rocket’s mishap was entirely random and it could have gone anywhere.

Long-period comets and odd objects like Sedna teach us much about what goes on outside the ecliptic, but most deep space missions that have commanded the public’s attention have had destinations somewhere within it. The two Voyagers have a more complicated story given their gravitational encounters, Voyager 1 having taken a jog at Saturn to fly by Titan and thus propel itself out of the ecliptic on an interstellar trajectory, while its twin, Voyager 2, left the system and ecliptic in another direction after its encounter with Neptune. Neither was designed for interstellar operations but both now comprise our only live craft beyond the heliosphere.

As our missions become still more ambitious, we push into this wider, spherical realm of reference, which inevitably shapes public attitudes about our relationship with the galaxy. New Horizons’ mission to Pluto reminds us that the dwarf planet is at a 17° tilt to the ecliptic. Going to other stars would shed this culturally embedded planar concept, for the most part, though it’s interesting that one nearby destination, Epsilon Eridani, lines up well enough with the ecliptic to offer a boost from the angular momentum available to a departing craft. Alpha Centauri, well south of the ecliptic, demands a trajectory bend that loses this bit of assistance. This is a point APL’s Ralph McNutt made to me almost 20 years ago, as I was reminded recently in going through my notes from that period.

Image: Voyager 1 and 2 trajectories. Voyager 1 visited Jupiter and Saturn, and then veered northward off of the plane of our solar system. Voyager 2 visited all four giant planets of the outer solar system before departing southward toward interstellar space. Credit: NASA.

When we start contemplating interstellar missions, we have the chance to do what Voyager did just once, to look back at the Solar System, but this time in a much broader context. The focus will not be on the planets and the pale blue dot of Earth, but rather on the heliosphere, from a vantage well beyond its outer regions. Interstellar Probe is a heliophysics mission in its attempt to understand the Sun and planets as a system moving through the interstellar medium. It pushes perspectives as we visualize the entire Solar System as a moving, interacting environment where life can emerge.

The burgeoning catalog of exoplanets clearly plays into the concept, for we see thousands of stellar systems, each with their own context in what we can call an ‘astrosphere.’ The host stars we study, a tiny fraction of the several hundred billion in the galaxy, all move through plasma and dust within the interstellar medium. We have little enough information about how the Sun’s solar wind carves out the magnetic bubble surrounding our Solar System, but about astrospheres around other stars, we know next to nothing. Our view is flattened; we see their planets, or their circumstellar disks, our instrumentation not up to the challenge of seeing an astrosphere.

Image: This is Figure 3-1 from the JHU/APL report on Interstellar Probe from 2019; the latest report will be out in December. Caption: As our type-G2V star plows through the galactic interstellar medium, it forms the habitable astrosphere harboring the entire solar system we live in. Of all other astrospheres, one of our habitable type has never been observed, and yet we are only at the very beginning of uncovering our own. An interstellar probe through the heliospheric boundary into the LISM would enable us to capture its global nature and would represent humanity’s first step into the galaxy, where unpredictable discoveries await. Credit: NASA/Rosine Lallement, 2020.

Make no mistake, the crossing of the heliopause by both Voyagers has supplied us with data on the plasma physics at work in this region, while from inside the heliosphere, missions like IBEX have revealed unusual features that demand clarification. Interactions at heliosphere’s edge involve solar plasma, and magnetic fields both solar and interstellar, as well as neutral particles in the medium and galactic cosmic rays. Charge-exchange processes between interstellar hydrogen atoms and solar plasma protons shape the heliosphere as does the solar magnetic field pervading it.

A mission that gets to a vantage as distant as 1000 AU will be able to see these interactions from the outside, to determine the heliosphere’s overall shape and the distribution of plasma within it, even as missions like the upcoming IMAP (Interstellar Mapping and Acceleration Probe) study the heliosphere’s boundary from well within it. A probe into the interstellar medium would allow us to examine how the Sun’s activity cycle affects the heliosphere’s recorded shock and pressure waves, as found in Voyager data. Voyager has also shown that the heliosphere shields the Solar System from approximately 75 percent of incoming galactic cosmic rays, a factor in habitability.

But back to movement through the medium. Many interstellar clouds are found in what is called the Local Bubble,a region of hot gas that extends several hundred light years from the Sun. The conception of the Solar System as moving through interstellar clouds of varying dust, plasma and gas content backs out the field of view yet again. The Sun moves at 26 kilometers per second toward the edge of the Local Interstellar Cloud and will exit it in about 1900 years, and the question of what cloud we move through next is open. Fifteen interstellar clouds have been identified within 15 parsecs of our system.

Our Voyagers will run out of power somewhere in the range of 160 AU from the Sun, a long way from what astronomers consider the undisturbed local interstellar medium. Putting a probe well beyond this range would provide the first sampling of the interstellar medium that is unaffected by the heliosphere, and thus teach us a great deal about what our solar bubble moves through. As interstellar dust grains are the foundation of both stellar and planetary systems, they hold clues to the formation of matter in the galaxy and the evolution of stars. All this is applicable, of course, not just to our own heliosphere but the astrospheres around exoplanetary systems.

Image: This is Figure 3-10 from the JHU/APL report. Caption: The Sun is on the way to exiting the Local Interstellar Cloud and entering another unexplored interstellar region. Credit: NASA/Goddard/Adler/U. Chicago/Wesleyan.

A mission designed to be returning data 50 years after launch, expressly interstellar in its conception, also elevates our thinking about time as we confront operations long after our own demise. Such a mission puts the blip of our present existence into the context of galactic rotation, the chronological equivalent of the pale blue dot image.

Deeper awareness of ourselves as part of a great astrophysical complex that renders life possible helps to place us in a galactic setting. Going interstellar demands looking a long way out, but it also demands looking back, in our data and imagery, to understand the bubble within which we emerged. That shift in perspective in turn feeds the interstellar ambition, as we expand the frame of reference to other stars.



Interstellar Reach: Exploration as Choice

Two missions with interstellar implications have occupied us in recent days. The first, Interstellar Probe, has significance in being the first dedicated mission into the local interstellar medium. Here the science return would be immense, as we would have the opportunity to view the heliosphere from the outside. Culturally, Interstellar Probe is the kind of mission that can force resets in how we view exploration, a thought I want to expand on in the next post.

The other mission — multiple mission options, actually — involves interstellar objects like the odd 1I/’Oumuamua and 2I/Borisov, the latter clearly a comet, the former still hard to categorize. In fact, between the two, what I think we can just call Comet Borisov seems almost pedestrian, with a composition so like comets in our own system as to suggest such objects are commonplace among the stars. Whereas to explain ‘Oumuamua as a comet, we have to stretch our definitions into bizarre objects of pure hydrogen (a theory that seems to have lost traction) or consider it a shard of a Pluto-like world made of nitrogen ice. We may never know exactly what it was.

The point of Andreas Hein and team was to show not just what might be capable with an all-out effort to catch ‘Oumuamua, but more important, to offer mission options for the next interstellar wanderer that makes its way through our system. Thus the implication for future interstellar activities is that we have the opportunity to study materials from another star long before we have the capability of putting human technologies near one. These objects become nearby, fast-moving destinations that form part of the morphology of our interstellar effort.

I use the term ‘morphology’ deliberately because of its dexterity. In linguistics, the study of a language’s morphology takes us deep into its internal structure and the process of word formation. In biology, the word refers to biological form and the arrangement of size, structure and constituent parts. Here I’m using it in a philosophical sense, to argue that we continually shape cultural expectations of exploration that govern what we are willing to attempt, and that doing this is an ongoing process that will decide whether or not we choose to move beyond Sol.

Going interstellar is a decision. It comes with no guarantees of success, but we know beyond doubt that only by learning what is possible and attempting it can we ever succeed.

It seems a good time to revisit an image of 2I/Borisov from the Hubble Space Telescope as we ponder strategies for future missions amidst these reflections. The instrument had been observing the comet since October of 2019, following its discovery by Crimean amateur astronomer Gennady Borisov in August of that year. The Hubble work revealed among other things the surprising fact that the comet turned out to be no more than about 975 meters across. This was unexpected, as David Jewitt (UCLA) explained at the time:

“Hubble gives us the best upper limit of the size of comet Borisov’s nucleus, which is the really important part of the comet. Surprisingly, our Hubble images show that its nucleus is more than 15 times smaller than earlier investigations suggested it might be. Our Hubble images show that the radius is smaller than half a kilometer. Knowing the size is potentially useful for beginning to estimate how common such objects may be in the solar system and our galaxy. Borisov is the first known interstellar comet, and we would like to learn how many others there are.”

All fodder for crafting mission concepts. The image below was taken in November of 2019. Here we have an interstellar interloper in our own system, framed along with the distant background spiral galaxy 2MASX J10500165-0152029. Notice the smearing of the galaxy image, a result of Hubble tracking the comet, which was at the time of image acquisition about 327 million kilometers from Earth. The blue color is artificial, used to draw out detail in the comet’s coma surrounding the nucleus (Credit: NASA, ESA and D. Jewitt (UCLA).

The immensity of the cosmos taunts us with our limitations, but in considering them, we choose directions for our thinking, our aspirations and our science. This image is emblematic. Out of the darkness comes something interstellar that we now believe is just one of many such objects open to investigation, and reachable by near-term technologies. A galaxy lies behind it. How far into our own galaxy can we push as our technologies morph into new capabilities?

Exploration is a decision. How far will we choose to go?



Reaching an Interstellar Interloper

The ongoing Interstellar Probe study at the Johns Hopkins University Applied Physics Laboratory reminds us of the great contribution of the Voyager spacecraft, but also of the need to develop their successors. Interstellar flight is a dazzling goal considered in the long term, but present technologies develop incrementally and missions to other stars are a multi-generational goal. But as we continue that essential effort with projects like Interstellar Probe, we can also make plans to explore objects from other stellar systems (ISOs) closer to home.

I refer of course to the appearance in the last three years of two such objects, 1I/’Oumuamua and 2I/Borisov, the ‘I’ in their names referencing the exciting fact that these are interstellar in nature, passing briefly through our system before moving on. Papers have begun to appear to examine missions to one or the other of these objects, or to plan how, with sufficiently early discovery, we could get a spacecraft to the next one. And keep in mind the ESA’s Comet Interceptor mission, which sets its sights on a long-period comet but could be used for an ISO.

Are missions to interstellar objects possible with near-term technology? A new paper from lead author Andreas Hein (Initiative for Interstellar Studies) and an international team of researchers answers the question in the affirmative. The paper characterizes such missions by the resources required to perform them, which in turn relates to the ISO’s trajectory. Unbound ISOs — those that pass through our system only once — can be contrasted with bound objects that have remained in the Solar System after their entry. If the ISO is unbound, a mission launched before perihelion would have the best chance of producing data and perhaps sample return.

Image: An artist’s impression of 2I/Borisov, an interstellar comet. Credit: NRAO/AUI/NSF, S. Dagnello.

In previous papers, Hein and team have considered chemical propulsion complemented by a reverse gravity assist at Jupiter and a Solar Oberth Maneuver to reach 1I/’Oumuamua, although they have also looked at thermal nuclear propulsion with gravity assist at Jupiter. Uncertainties in the object’s orbit are challenging but, the authors believe, surmountable through the use of a telescope like that of New Horizons (LORRI) or, a highly speculative idea, a swarm of chipsats that could be launched ahead of the probe to refine navigational data. This approach goes well beyond existing technology, though, as the authors acknowledge by citing the work on Breakthrough Starshot’s laser architecture, which is a long way from realization.

I’m also concerned about that notion of a Solar Oberth Maneuver, given what we’ve learned recently in connection with the research on Interstellar Probe, for the kind of spacecraft described here to intercept 1I/’Oumuamua would carry the needed upper stage kick engine, along with the heat-shield technology Interstellar Probe has been investigating. All this adds to mass. The authors believe Falcon Heavy (or, unlikely, a future SLS) would be up to the challenge, but I think the proposed Solar Oberth Maneuver at 6 solar radii is a problematic goal in the near-term.

The authors echo these sentiments in terms of the perihelion burn itself as well as the navigation issues to reach the ISO which will ensue. A propulsive burn at perihelion for a probe trying to intercept an interstellar object is a long way from proven technology, particularly when we’re hoping to deliver a substantial instrument package to the ISO for science return. The authors call for developing nuclear thermal propulsion in order to make a wider range of ISOs reachable without relying on the Oberth maneuver.

The paper usefully offers a taxonomy of interstellar objects, matched to their associated science and conceivable mission types. Objects with low inclinations, low hyperbolic escape velocity (v), and those discovered well before perihelion are the most reachable targets. Of course, this survey of options for reaching an ISO isn’t intended to be specific to a given object but applicable to many, suggesting what is possible with present and near-term technologies. In the discussion of a mission to 1I/’Oumuamua, the authors also note the wide range of details that need to be considered:

Our brief analysis (and its attendant caveats) should not be regarded as exhaustive. Other issues that we have not delineated include the difficulties posed by long CCD exposure times (11 hours in our scenario) such as the cumulative impact of cosmic rays and the necessity of accounting for parallax motion of the object during this period. Obstacles with respect to measuring the position of the object, calculating offsets, and relaying it to the spacecraft may also arise. Hence, we acknowledge that there are significant (but not necessarily insurmountable) and outstanding challenges that are not tackled herein, as they fall outside the scope of this particular paper.

In any event, 1I/’Oumuamua may be quite a tricky object to catch at this juncture even for this kind of fast flyby. Objects detected earlier in their entry into our system should present a much more workable challenge, and with the Vera Rubin Observatory coming into play, we are probably going to be finding many more of them, some well before perihelion. Hence the need to know what is possible for future operations at ISOs, ensuring we have a plan and resources available to fly when we next have the opportunity.

A rendezvous mission may one day be in the cards, with the authors relying on electric or magnetic sail propulsion schemes to allow the spacecraft to slow down and study the target at close hand. But it may be more reasonable to consider rendezvous with captured interstellar objects in bound elliptical orbits. These are missions which are examined here in relation to two potential ISOs (not yet confirmed as such), (514107) Ka’epaoka’awela, a Jupiter co-orbital in retrograde orbit, and the Centaur 2008 KV42. The paper examines rendezvous strategies and provides trajectories for multiple years. 2008 KV42, for example, should be reachable for rendezvous with launch in 2029 and a flight duration of 15 years.

Finally, nuclear thermal technologies should allow sample return from some interstellar objects using a pre-positioned interceptor at the Sun/Earth L2 point. The paper considers an interceptor mission to comet C/2020 N1, serving as a surrogate for particular types of ISOs. The spacecraft, using nuclear thermal or solar electric propulsion, would deploy an impactor on approach to the object and travel through the plume, perhaps using swarm subprobes to return samples to the main craft depending on whether or not the plume is thought likely to be hazardous.

Even without nuclear thermal capability, though, missions can be flown to some types of interstellar objects with technologies that are currently in use. From the paper:

Our results indicate that most mission types elucidated herein, except for sample return, could be realized with existing technologies or modified versions of existing technologies, such as chemical propulsion and a Parker Solar Probe-type heat shield (Hein et al., 2019; Hibberd et al., 2020). Collisions with dust, gas, and cosmic rays and spacecraft charging in the interplanetary or interstellar medium will engender deflection of the spacecraft trajectory and cause material damage to it, but both effects are likely minimal even at high speeds (Hoang et al., 2017; Hoang & Loeb, 2017; Lingam & Loeb, 2020, 2021), and the former can be corrected by onboard thrusters.

So we learn that missions to interstellar objects are feasible, with some fast flyby scenarios capable of being accomplished with today’s technologies. Rendezvous and sample return missions await the maturation of solar electric and nuclear thermal propulsion. Here the concept ‘near-term’ is speculative. When will we have nuclear thermal engines available for this kind of mission? I am speaking in a practical sense — we know a great deal about nuclear thermal methods, but when will we deploy workable engines at a high enough Technology Readiness Level to use?

There is much we could learn from an ISO intercept, whether a flyby, a rendezvous or a sample return. Given that we are a long way from being able to sample interstellar objects in other stellar systems (I doubt seriously we’ll have this capability in a century’s time), ISOs represent our best bet to discover the structure and composition of extrasolar objects. This and the capability of doing interplanetary dust and plasma science along the way should be enough to keep such missions under active study as our new generation telescopes come online.

The paper is Hein et al., “Interstellar Now! Missions to Explore Nearby Interstellar Objects,” in press at Advances in Space Research (abstract / preprint).



Assessing the Oberth Maneuver for Interstellar Probe

I notice that the question of ‘when to launch’ has surfaced in comments to my first piece on Interstellar Probe, the APL study to design a spacecraft that would be, in effect, the successor to Voyager. It’s a natural question, because if a craft takes 50 years to reach 1000 AU, there will likely be faster spacecraft designed later that will pass it in flight. I’m going to come down on the side of launching as soon as possible rather than anticipating future developments.

Two reasons: The research effort involved in stretching what we can do today to reach as high a velocity as possible inevitably moves the ball forward. We learn as we go, and ideas arise in the effort that can hasten the day of faster spacecraft. The second reason is that a vehicle like Interstellar Probe is hardly passive. It does science all along its route. By the time it reaches 1000 AU, it has returned massive amounts of information about the interstellar medium, our Sun’s passage through it, and the heliosphere that protects the Solar System.

All of that is germane to follow-on missions, and we have useful science data all the way. So I’m much in favor of pushing current technology into stretch missions even as we examine how to go faster still with the next iteration, the one that would succeed Interstellar Probe.

Getting Up to Speed

How fast can we travel now, as compared to 1977, when we launched Voyagers 1 and 2? We know we can reach 17 kilometers per second with 1977 technology because that is what Voyager 1 is doing right now. Interstellar Probe advocates would like to see something in the range of 95 kilometers per second as a way of making the 1000 AU journey in 50 years. That’s still, I suppose, within the lifetime of a researcher, but not by much, and it’s heartening to me that we’re extending the boundaries into a frank admission of the fact that some missions may be launched by one generation, maintained by another, and brought home by a third.

I always assumed we had an ace up our sleeves when it came to ramping up Voyager speed levels. Moving close to the Sun and making a propulsive burn at just the right moment seemed a sure way to exploit that deep gravity well and fling a probe outward at high velocity. The idea first appeared in Hermann Oberth’s Wege zur Raumschiffahrt (Paths to Spaceflight), which was published in 1929 in Germany. At the time, Oberth was also working as a consultant on the Fritz Lang film Frau im Mond (The Woman in the Moon), which would popularize the idea of rocketry and space travel. In fact, Oberth would dedicate Wege zur Raumschiffahrt to Lang and actress and screenwriter Thea von Harbou.

The authors of the Interstellar Probe 2019 report note in their extremely useful appendices that Oberth’s thinking on the maneuver that would be named after him anticipated in many ways the idea of using a gravity assist that was developed in the 1960s by Michael Minovitch. His thought experiment involved an astronaut on an asteroid 900 AU from the Sun. The astronaut, apparently quite long-lived, wants to go to a star some 1015 kilometers away (roughly the distance of Regulus). His asteroid has an orbital speed of 1 km/s and an orbital period of 27,000 years.

I won’t go into this in huge detail because it’s laid out so well in the report’s appendix (available here). But Oberth’s setup is that the target star is in the orbital plane of the asteroid, and he assumes the astronaut has a rocket that can produce a velocity change of 6 km/s. Sun, asteroid and target star are in a line in that order. He asks: What is the fastest way to reach the star?

Using the rocket alone reaches it in 5,555,000 years. Waiting for 20,000 years to add the asteroid’s orbital velocity to the velocity of the ship reduces that to 4,760,000 years. But Oberth realizes that the best answer is to use the rocket to move opposite to the asteroid’s motion, falling in toward the Sun to reach 500 km/s at perihelion, then using the remaining rocket fuel to boost the speed a bit further. He ultimately gets 70.9 km/s moving out of the Solar System, and his transit time is now reduced to 470,000 years. Thus the ‘Oberth maneuver’ enters the literature.

A spacecraft launched from Earth has to lose the heliocentric angular momentum of Earth’s orbit to fall toward the Sun in order to make the Oberth maneuver possible, the most efficient method being a direct trajectory from Earth to Jupiter, a retrograde gravity assist at Jupiter, and a long fall back to perihelion, at which point a kick-stage provides the further propulsive burn. All of this, including of course the thermal issues raised by putting the payload into such proximity to the Sun, has to be weighed against a straight gravity assist at Jupiter, with no close solar pass, when contemplating how best to accelerate the Interstellar Probe for the journey.

Image: This is an image of Parker Solar Probe as envisioned by Goddard Media Studios at NASA’s Goddard Space Flight Center in Maryland. It’s the closest I could come to what a close solar pass would look like, though it lacks the propulsive element of the Oberth maneuver. Credit: NASA GSFC.

Oberth in Today’s Terms

When I contacted Interstellar Probe principal investigator Ralph McNutt (JHU/APL) about these issues, he pointed out that the Mission Concept Report for the entire project would be made available on the probe website in the first week of December. Putting what the report will describe as the Solar Oberth Maneuver (SOM) through the severe filter of engineering capabilities with today’s technologies is a major priority of this report, and the results McNutt conveyed make it clear that my enthusiasm for the concept has been unjustified.

Unjustified, that is, in terms of a spacecraft being designed, as this one must be, around current technologies. Remember that we’re talking about a mission with a specific timeframe, one with a launch in the early 2030s, meaning that the materials and techniques to build and fly it have to be within range today. The Oberth maneuver at the Sun may have possibilities for us down the road. But today’s engineering constraints make the issues stark. As McNutt told me in an email:

…after a very careful look and relying on the same people, including the mission system engineer, who worked the thermal protection system (TSA) for Parker Solar Probe (PSP) we have concluded (1) the SOM offers no advantage over prograde gravity assists in rapid escape from the solar system for a “technology horizon” in the 2030’s and (2) there is no obvious “path” to changing this conclusion for the foreseeable future.

Image: Ralph L. McNutt Jr., chief scientist for Space Science at the Johns Hopkins University Applied Physics Laboratory and principal investigator for Interstellar Probe. Credit: Johns Hopkins University.

In other words, going to Jupiter straightaway, with no Oberth maneuver, is just as workable, and as we’ll see, avoids a series of thorny problems. One issue is the need for thermal protection, another the demand of launching a payload sufficiently large, one that would incorporate not only the propulsive stage for operations at perihelion preceding the long cruise, but would also include the science instrument package and the necessary high gain antenna that would be needed for data downlink at the distances the probe is envisioned to reach. We have to work within the constraints of present-day launch systems as well as existing engines for the kick.

On thermal issues, the Interstellar Probe team worked with Advanced Ceramic Fibers, an Idaho-based company, on ultra-high temperature material studies, the question being how one could take existing thermal protection as found on the current Parker Solar Probe mission and extend it into the range needed for the Solar Oberth Maneuver. But shield mass, said McNutt, is only one consideration. A ‘ballast’ mass is also required to keep the center of gravity moving along the engine centerline as the propellant burns down during the maneuver.

These issues of mass are critical. Let me quote McNutt again:

The real problem is the mass of the thermal shield assembly – multiple shields plus the supporting structure – to shield just the kick stage itself, even with no Interstellar Probe spacecraft. We’ve adopted solid rocket motors (SRMs) with specific impulses approaching 300s with masses of up to ~4,000 kg (Orion 50XL). In that case, we have an engineering solution that closes on paper, has all of the design margins included, would require specialized design work (> ~10’s of millions and multiple years of dedicated effort) and ends up with about the same performance (flight distance after 50 years) as a prograde Jupiter gravity assist, but with significantly more inherent risk, both in development and in the actual execution of the burn at the Sun itself. Bottom line: it may be doable with an investment of significantly more time and money, but it offers no advantage, and, therefore, we have concluded it would be a poor trade.

Within the upcoming report will be the 181 staging scenarios the team examined by way of reaching its conclusions about the Solar Oberth Maneuver. It becomes clear from the synopsis that McNutt gave me that existing technologies are simply not up to speed to realize the potential of the SOM, and even extending the technologies forward to nuclear rocket engines and greatly enhancing the performance of today’s launch vehicles would not change this fact. To make the Oberth maneuver at the Sun into a viable option, it appears, would take decades of work and demand billions of dollars in new investment. Best to shelve Oberth’s concept for this mission, though I suspect that future technologies will keep the concept in play.

Where to next with Interstellar Probe? If we rule out Oberth, then the two scenarios involving a Jupiter gravity assist remain, the team having considered other options including solar sails and finding them not ready within the needed timeframe. The first is a ‘passive’ flyby, in which every rocket stage is fired in an optimized launch sequence. The second is a powered gravity assist, in which a final kick-stage is reserved for use at Jupiter. We will see what the upcoming report has to say about these options, balancing among outbound speed, complexity, and mass.



Interstellar Probe: Pushing Beyond Voyager

Our doughty Voyager 1 and 2, their operations enabled by radioisotope power systems that convert heat produced by the decay of plutonium-238 into electricity, have been pushing outward through and beyond the Solar System since 1977. Designed for a four and a half year mission, we now have, more or less by accident and good fortune, our first active probes of nearby interstellar space. But not for long. At some point before the end of this decade, both craft will lack the power to keep any of their scientific instruments functioning, and one great chapter in exploration will close.

What will the successor to the Voyagers look like? The Johns Hopkins University Applied Physics Laboratory (JHU/APL) has been working on a probe of the local interstellar medium. We’re talking about a robotic venture that would be humanity’s first dedicated mission to push into regions that future, longer-range interstellar craft will have to cross as they move far beyond the Sun. If it flies, Interstellar Probe would be our first mission designed from the start to be an interstellar craft.

Pontus Brandt is an Interstellar Probe Concept Study project scientist, in addition to being principal investigator for two instruments aboard the European Space Agency’s Jupiter Icy Moon Explorer (JUICE) Mission. Brandt puts the ongoing work in context in a recent email:

Interstellar Probe would represent Humanity’s first deliberate step into interstellar space and go farther and faster than any spacecraft before. By using conventional propulsion, Interstellar Probe would travel through the boundaries of the protective heliosphere into the unknown interstellar cloud for the first time. Within its lifetime, it would push far beyond the Voyager mission to explore the heliospheric boundary and interstellar space so that we can ultimately understand where our home came from, and where we are going.

Image: A possible operation scenario, divided into phases and indicating science goals along the way. Credit: JHU/APL, from the Interstellar Probe 2019 Report.

The nature of the interstellar cloud Brandt refers to is significant. But before examining it, a bit of background. APL’s role in Interstellar Probe has roots in principal investigator Ralph McNutt’s tireless advocacy of what was once called Innovative Interstellar Explorer, a report originally funded by NASA in 2003 and often discussed in these pages. The current study began in 2018 and will continue through early 2022, examining the technologies that would make Interstellar Probe possible, with an eye on the coming Decadal Survey within NASA’s Heliophysics Science Division. Bear in mind as well that the space community has been discussing what we can call ‘interstellar precursor’ missions all the way back to the 1960s — an interesting story in itself! — and the Interstellar Probe concept appeared in the 2003 and 2013 Heliophysics Decadal Surveys.

About those Decadals: Every ten years, Decadal Surveys appear for the four NASA science mission divisions: Planetary Science, Astrophysics, Heliophysics and Earth Science, the idea being to provide guidance for the agency’s science program going forward. So the immediate context of the current effort at APL is that it is being conducted to provide technical input that can feed into the next Heliophysics Decadal Survey, which will cover the years 2023 to 2032. But the implications for science across all four divisions are part of APL’s remit, affecting specific targets and payloads.

What can realistically be done within the 2023-2032 time frame? And what kind of science could a mission like this, launching perhaps in 2030, hope to accomplish? Workshops began in June of 2018 and continue to refine science goals and support engineering trade studies in support of what the team calls “a ‘pragmatic’ interstellar probe mission.” The most recent of these, the fourth, just concluded on October 1. You can see its agenda here.

A launch in the early 2030s demands not futuristic technologies now in their infancy but proven methods that can be pushed hard in new directions. This is, you might say, ‘Voyager Plus’ rather than the starship Enterprise, but you build interstellar capability incrementally absent unexpected breakthroughs. That calls for a certain brute force determination to keep pushing boundaries, something Ralph McNutt and team have been doing at APL, to their great credit , for many years now. A spacecraft like this would be a flagship mission (now known as a Large Strategic Science Mission) — these are the most ambitious missions the agency will fly, a class that has included the Voyagers themselves, Cassini, Hubble and the James Webb Space Telescope.

A variety of methods for reaching beyond the heliosphere in the shortest possible time have been under consideration, including an “Oberth maneuver” (named after scientist Hermann Oberth, who documented it in 1929), where a propulsive burn is performed during a close solar pass that has itself been enabled by a retrograde Jupiter gravity assist. Other Jupiter flyby options, with or without a propulsive burn via a possible upper stage, remain on the table. The plan is to drive the probe out of the Solar System at speeds sufficient to reach the heliopause in 15 years. The participating scientists talk in terms of a flyout speed of 20 AU/year, which translates to 95 kilometers per second. Voyager 1, by comparison, is currently moving at roughly 17.1 kilometers per second.

The Voyagers own our current distance records, with Voyager 1 currently at 154 AU and Voyager 2 at 128 AU. Interstellar Probe would still be returning science at 1000 AU, meaning it would be capable of looking back and seeing not just the Earth in the context of the Solar System, as in Voyager’s ‘pale blue dot’ image, but also taking measurements of the heliosphere from well outside it, helping us understand both the interstellar medium and the effect of our stellar system as it moves through it.

There is much to be learned about the protective magnetic bubble called the heliosphere in which the entire Solar System is embedded. We have to understand that it is anything but static, as Pontus Brandt explains:

During its evolutionary journey around the galaxy, [the Sun] has plowed through widely different environments, witnessed supernova explosions on its path, that have all shaped the system that we live in today. The vast differences in interstellar densities, speeds, charge fractions have been responsible for an extreme range of sizes and shapes of the global heliosphere throughout its history – from many times bigger than today, to a tiny heliosphere below even the orbit of Earth. This, in turn, has had dramatic consequences for the penetration of the primordial soup of interstellar material that have affected several crucial aspects of elemental and isotopic abundances, atmospheric evolution, conditions for habitability and perhaps even biological evolution. Only some 60, 000 years ago, the Sun entered the vast Local Interstellar Cloud (some 30 light years across), and in just a few thousand years the solar system will enter a completely different interstellar cloud that will continue to shape its evolution.

Image: The Sun is on the way to exiting the Local Interstellar Cloud and entering another unexplored interstellar region. Credit: NASA/Goddard/Adler/U. Chicago/Wesleyan.

An interstellar precursor mission can examine energetic neutral atoms (ENAs) to provide data on the overall shape of the heliosphere. Major issues include how plasma from the Sun’s solar wind interacts with interstellar dust to form and continue to shape the heliosphere.

But a mission like this also shapes our views of time, as the Voyagers have done as we have watched their progress through the Solar System, the heliosphere and beyond. Mission scientists turned the 4.5 year mission into a surprising 45 year one solely on the strength of their design and the quality of their components, not to mention the unflagging efforts of the team that operates them. A mission designed from the start for 50 years, as Interstellar Probe would be, will likely have a lifetime far beyond that. Its components are meant to be functional when our grandchildren are in their dotage. Most of its controllers in 2080 have yet to be born.

So this is a multi-generational challenge, a reach beyond individual lifetimes. Let me quote from the Interstellar Probe Study 2019 Report, which is now available online.

It is important to note that the study does not purport to center on “the one and only” interstellar probe but rather on this mission as a first step to more advanced missions and capabilities… In addition to promising historically groundbreaking discoveries, the Interstellar Probe necessitates a transformation in the programmatics needed to accommodate lifetime, reliability, and funding requirements for this new type of multigenerational, multi-decade operational mission. Paving the way for longer journeys utilizing future propulsion technologies, such as those not invoked here, the Interstellar Probe is the first explicit step we take today on the much longer path to the stars.

Principal investigator Ralph McNutt tells me that the Interstellar Probe team is finishing up a Mission Concept Report for NASA on the progress thus far, incorporating results of the recent workshop. This report should be available on the Interstellar Probe website in early December, with a number of items clarifying aspects of the currently available 2019 report. We need to dig into some of the issues that will appear there, for the concept is changing as new studies emerge. In particular, let’s look next time at the ‘Oberth maneuver’ idea, what it means, and whether it is in fact a practical option. I’m surprised at what’s emerging on this.



The Survival of M-Dwarf Planet Atmospheres

I was interested in yesterday’s story about the two super-Earths around nearby M-dwarfs — TOI-1634b and TOI-1685b — partly because of the research that follows. In both cases there is the question of atmospheres. The two TESS planets are so numbingly close to their host stars that they may have lost their original hydrogen/helium atmospheres in favor of an atmosphere sustained by emissions from within. Hearteningly, we should be able to find out more with the James Webb Space Telescope, on which ride the hopes of so many exoplanet researchers.

Today’s system is the intriguing L 98-59, only 35 light years from Earth and possessed of at least four planets, with a fifth as yet unconfirmed. Here we have two rocky inner worlds, a possible ocean planet (L 98-59 d) and another likely rocky world to the inside of the habitable zone boundary. Perhaps within the habitable zone, if it exists, is L 98-59f, so this is a system to keep an eye on, an obvious candidate as a JWST target.

At UC Riverside, Daria Pidhorodetska wants to know whether small, rocky planets orbiting M-dwarfs like this one have atmospheres. The paper is devoted to the question of whether either Hubble or JWST, perhaps working in tandem, could detect atmospheres in this system. The authors proceeded to model four different types of atmospheres to answer the question with regard to the three inner planets.

Image: This is an infographic from the European Southern Observatory showing a comparison between the L 98-59 exoplanet system (top) with part of the inner Solar System (Mercury, Venus and Earth), highlighting the similarities between the two. L 98-59 contains four confirmed rocky planets (marked in color in the top panel), orbiting a red-dwarf star 35 light-years away. The planet closest to the star is around half the mass of Venus, making it the lightest exoplanet ever detected using the radial velocity technique. Up to 30% of the third planet’s mass could be water, making it an ocean world. The existence of the fourth planet has been confirmed, but scientists don’t yet know its mass and radius (its possible size is indicated by a dotted line). The team also found hints of a potential fifth planet, the furthest from the star, though the team knows little about it. If confirmed, it would sit in the system’s habitable zone where liquid water could exist on its surface. Credit: ESO/L. Calçada/M. Kornmesser (Acknowledgment: O. Demangeon). [Note: The distances from the stars and between the planets in the infographic are not to scale. The diagram has been scaled to make the habitable zone in both the Solar System and in L 98-59 coincide].

A major problem for M-dwarf planets, to go along with tidal lock, is the fact that during their formation, they are bathed in intense ultraviolet radiation. Enough so that the potential is there to cause any water at the surface to evaporate, while their atmospheres would be under a fierce barrage and might not survive. The question for Pidhorodetska and team is, then, whether the two inner rocky planets have lost their atmospheres completely, if they had one, or if they have been able to replenish them.

The range of atmospheric scenarios takes in planets with atmospheres dominated by water, hydrogen, carbon dioxide, or oxygen and ozone (remaining after loss of hydrogen). The authors argue that an oxygen-dominated atmosphere is the most likely. For each of these scenarios, the authors simulated transmission spectroscopy. L 98-59’s proximity to Earth as well as the fast orbits (less than a week) of its planets speeds up the process of discovery. In fact, says Edward Schwieterman (UC Riverside):

“It would only take a few transits with Hubble to detect or rule out a hydrogen- or steam-dominated atmosphere without clouds. With as few as 20 transits, Webb would allow us to characterize gases in heavy carbon dioxide or oxygen-dominated atmospheres.”

I’m interested, though, specifically in that question of atmosphere loss, with hydrogen escape leaving oxygen and ozone behind. The paper explains:

Highly irradiated planets such as those of the L 98-59 system could have a desiccated atmospheric composition, such as one that is dominated by O2, as a result of major ocean loss during an extended runaway greenhouse phase. A desiccated planet that is rich in abiotic O2 would be expected to form O3 from the photochemical processing of O2, meaning that the direct detection of O3 absorption could be another key indicator of this planetary state.

And the authors point out in their conclusion that we can learn a great deal about the evolution of these planets depending on whether we detect water in their atmospheres. An atmosphere high in oxygen due to the loss of hydrogen during the star’s pre-main sequence phase — in other words, an atmosphere that survives utter desiccation — should have no oceans to detect. Water, or the lack of it, is another marker for this early stage of planetary evolution, and our instruments should be able to make the call.

The paper is Pidhorodetska et al., “L 98-59: A Benchmark System of Small Planets for Future Atmospheric Characterization,” Astronomical Journal Vol. 162, No. 4 (29 September 2021), 169 (full text).



Atmospheric Evolution on Hot Super-Earths

Hot Jupiters (notice I’ve finally stopped putting the term into quotation marks) were the obvious early planets to detect, even if no one had any idea whether such things existed. I suppose you could say Greg Matloff knew, at least to the point that he helped Buzz Aldrin and John Barnes come up with a plot scenario involving a planet that fit the description in their novel Encounter with Tiber (Grand Central, 1996), which was getting published just as the hot Jupiter 51 Pegasi b was being discovered. Otto Struve evidently predicted the existence of gas giants close to their star as far back as 1952, but it’s certainly true that planets like this weren’t in the mainstream of astronomical thinking when 51 Pegasi b popped up.

Selection effect works wonders, and it makes sense that radial velocity methods would bear first fruit with a large planet working its gravitational effects on the star it orbits closely. Today, using transits, gravitational microlensing, astrometry and even direct imaging, we’re uncovering a much more representative sample of what’s out there, and hot Jupiters are somewhere around 1 percent of the catalog. Similarly rare, I’m sure, are the planets classed as ultra-short period worlds (USP), though they’re much smaller, with radii in the range of 2 Earth radii and periods of less than a day. We now have evidence for somewhere around 100 of these planets.

Two recently discovered examples are the planet candidates TOI-1634b and TOI-1685b, both of them TESS catches and subsequently examined by the Subaru Telescope (Mauna Kea, Hawaiʻi), supplemented with data from other observatories. An international team of astronomers have been at work on this duo, led by Teruyuki Hirano (University for Advanced Studies, Tokyo). The planet candidates are in Perseus, the first about 114 light years away; the second is 122 light years out. Both have now been confirmed to be rocky super-Earths (and I’m no longer going to put that term in quotation marks either). Both are likewise USPs, with ultra-tight orbits of less than 24 hours.

Image: Artist’s conceptual image showing the sizes of the planets observed in this study. The radius of TOI-1634b is 1.5 times larger than Earth’s radius and TOI-1685b is 1.8 times larger [note: the planets are mislabeled in the image]. The planets would appear red, due to the light from the red dwarf stars they orbit. Credit: Astrobiology Center.

As you’ll note from the caption above, the Subaru work (using the InfraRed Doppler — IRD — spectrograph mounted on the telescope) has been able to measure the masses of these transiting worlds. Here the chief interest is the fact that both lack a primordial hydrogen-helium atmosphere, which may or may not be the result of their proximity to their host stars.

Are there secondary atmospheres here, made up of gases released from within the planets? It’s an interesting speculation, as examining their constituents would tell us a lot about how these planets formed. The origins of USPs have been discussed in the literature in terms of inward migration, their highly circularized orbits the result of tidal interactions. There has been some suggestion that they represent remnant cores from hot Jupiters whose atmospheres have dissipated, but their hosts stars have different metallicity values than hot Jupiter host stars.

TOI-1634b and TOI-1685b, both of them orbiting M-dwarfs and relatively close to the Earth, may help us move forward in probing these issues, because to date there are few well characterized planets in the USP category; in fact, only the red dwarfs LTT 3780 and GJ 1252 have USP planets whose masses have been precisely measured. The authors note that TESS has been deeply involved in the hunt for more USPs, with 151 candidates announced as of February of 2021. 31 of these are known to orbit M-dwarfs.

The authors drew data from instrumentation ranging from Subaru itself to the MuSCAT imager at the Okayama Astro-Complex in Japan, the TCS telescope at Teide Observatory in Spain, the Gemini North telescope and Keck Observatory on Mauna Kea. Thus we’re examining data from ground-based transit photometry to high-resolution imaging, reconnaissance spectroscopy and radial velocity measurements, refining the orbital periods of these planets by more than an order of magnitude. Their future utility in atmospheric studies is clear. From the paper:

TOI-1634b is one of the largest and most massive USP planets having an Earth-like composition, and therefore, would become a benchmark target to study the formation and evolution history of massive USP planets. Both planets are listed among the best suitable targets for future atmospheric studies of small rocky planets by emission spectroscopy thanks to the brightness of the host stars, which encourages future characterizations using large aperture telescopes including JWST. Although small USP planets (< 2 R) are likely to have lost the primordial atmospheres dominated by H2 and He, one may be able to probe and constrain the secondary atmosphere formed via the outgassing from the planet interior.

The paper is Hirano et al, “Two Bright M Dwarfs Hosting Ultra-Short-Period Super-Earths with Earth-like Compositions,” The Astronomical Journal Vol. 162, No. 4 (23 September, 2021). Abstract / Preprint.



Hit-and-Run: Earth, Venus and Planet-Shaping Impacts

The gradual accretion of material within a protoplanetary disk should, in conventional models, allow us to go all the way from dust grains to planetesimals to planets. But a new way of examining the latter parts of this process has emerged at the University of Arizona Lunar and Planetary Laboratory in Tucson. There, in a research effort led by Erik Asphaug, a revised model of planetary accretion has been developed that looks at collisions between large objects and distinguishes between ‘hit-and-run’ events and accretionary mergers.

The issue is germane not just for planet formation, but also for the appearance of our Moon, which the researchers treat in a separate paper to extend the model for early Earth and Venus interactions that appears in the first. In the Earth/Venus analysis, an impact might be a glancing blow that, given the gravitational well produced by the Sun, could cause a surviving large part of an Earth-impactor (the authors call this a ‘runner’) to move inward and subsequently collide with Venus. So we’re not talking about impacts alone, but about impact ‘chains.’ The implications of this multi-impact theory on planet composition may be profound.

Alexandre Emsenhuber (now at Ludwig Maximilian University, Munich) is lead author of the paper on Earth/Venus interactions, pointing to the different impact scenarios for Earth and Venus:

“The prevailing idea has been that it doesn’t really matter if planets collide and don’t merge right away, because they are going to run into each other again at some point and merge then. But that is not what we find. We find they end up more frequently becoming part of Venus, instead of returning back to Earth. It’s easier to go from Earth to Venus than the other way around.”

Image: The terrestrial planets of the inner solar system, shown to scale. According to ‘late stage accretion’ theory, Mars and Mercury (front left and right) are what’s left of an original population of colliding embryos, and Venus and Earth grew in a series of giant impacts. New research focuses on the preponderance of hit-and-run collisions in giant impacts, and shows that proto-Earth would have served as a ‘vanguard’, slowing down planet-sized bodies in hit-and-runs. But it is proto-Venus, more often than not, that ultimately accretes them, meaning it was easier for Venus to acquire bodies from the outer solar system. Credit: Lsmpascal – Wikimedia Commons.

This work draws on a 2019 analysis by the same authors that first examined hit-and-run collisions and subsequent mergers of the two bodies. The authors point out that most simulations of this stage of planetary evolution assume perfect mergers for all impacts that are not completely catastrophic. Reflecting on this, they write:

Emsenhuber & Asphaug (2019a, hereafter Paper I) showed that this is not generally the case. They studied the fate of the runner following hit-and-runs into proto-Earths at 1 au, for thousands of geometries, and found that, contrary to expectation, only about half the time (depending on the runner’s egress velocity, which depends on the impact velocity and angle) do they return to collide again with proto-Earth. When they do, the return collision happens on a timescale of thousands to millions of years.

That work — fully treated in the first of the papers cited below — also revealed that the majority of the runners that did not return to the forming Earth would be likely to collide with Venus, given the assumption of their current masses and orbits. Those runners that did return would show an impact velocity in the second collision similar to the egress velocity after the first hit and run, thus slower than the original impact because of momentum loss. Follow-on collisions, then, are likely to be slow.

So we have a scenario in which the Earth takes repeated hits and spins off many impactors toward the inner system as they fall deeper into the Sun’s gravity well rather than eventually assimilating them itself. It’s an interesting notion given that, while Earth and Venus (so-called ‘sister planets’) have similar mass and density, Venus is nonetheless in a distinctly different state, its rotation retrograde compared to other planets, with a single rotation taking 243 days. There are also no moons at Venus. Do impacts during formation account for the differences?

To put the thesis to the test, the scientists built predictive models from 3D simulations of such impacts, drawing on machine learning techniques. They simulated terrestrial planet evolution over the course of 100 million years, calculating both hit-and-run collisions and those in which the impactor merged with the object struck.

The simulations explore the dynamical evolutions of remnants of hit-and-run collisions until the impactor is finally accreted or ejected.The different scenarios, says Asphaug, portray a sharply different formation history for the two worlds:

“In our view, Earth would have accreted most of its material from collisions that were head-on hits, or else slower than those experienced by Venus. Collisions into the Earth that were more oblique and higher velocity would have preferentially ended up on Venus…. We find that most giant impacts, even relatively ‘slow’ ones, are hit-and-runs. This means that for two planets to merge, you usually first have to slow them down in a hit-and-run collision. To think of giant impacts, for instance the formation of the moon, as a singular event is probably wrong. More likely it took two collisions in a row.”

Image: The Moon is thought to be the aftermath of a giant impact. According to a new theory, there were two giant impacts in a row, separated by about 1 million years, involving a Mars-sized ‘Theia’ and proto-Earth. In this image, the proposed hit-and-run collision is simulated in 3D, shown about an hour after impact. A cut-away view shows the iron cores. Theia (or most of it) barely escapes, so a follow-on collision is likely. Credit: A. Emsenhuber/University of Bern/University of Munich.

Earth’s impact history thus has a telling influence on planetary composition. From the paper:

…if the terrestrial planets formed in multiple giant impacts, then Venus is significantly more likely than Earth to have accreted a massive outer solar system body during the late stage of planet formation. Earth, by contrast, has no terrestrial planet beyond its orbit to act as a vanguard. Mars is about the same mass as the late-stage projectiles…, 0.1 M, and thus relatively inconsequential in terms of slowing them down through hit-and-run, so Earth has to do it on its own.

The late stage of terrestrial planet evolution in our own Solar System thus may hinge on how each world dealt with these impact runners. One thing that emphatically emerges from the work is that, according to these simulations, the terrestrial planets were hardly isolated during this period. Hit-and-run objects strike one planet, then the other, the probability of the impacts factored into the simulation via relative velocity and orbital configuration choices in the analysis.

In this study, Earth slows down projectiles, but accretes no more than half of them itself. Venus becomes a sink for these objects, retaining the majority of them in all simulations after their encounter with Earth as the slowed velocity of the runner allows for subsequent accretion. This would naturally lead to differences in composition between Venus and Earth and would account for differences in everything from Venus’ spin state, its formation (or lack of it) of moons, to its core-mantle dynamics. The authors promise a follow-up paper exploring these issues.

The papers are Emsenhuber et al., “Collision Chains among the Terrestrial Planets. II. An Asymmetry between Earth and Venus,” Planetary Science Journal Vol. 2, No. 5 (23 September, 2021), 199 (full text). The second paper is Asphaug et al., “Collision Chains among the Terrestrial Planets. III. Formation of the Moon,” Planetary Science Journal Vol. 2, No. 5 (23 September, 2021), 200 (full text)