Centauri Dreams
Imagining and Planning Interstellar Exploration
On 300 Million Habitable Zone Planets
We’ve talked about the Drake Equation a good deal over the years, but I may not have mentioned before that when Frank Drake introduced it in 1961, it was for the purpose of stimulating discussion at a meeting at the National Radio Astronomy Observatory in Green Bank, West Virginia that was convening to discuss the nascent field of SETI.
This was in the era of Drake’s Project Ozma and the terms of the SETI debate were hardly codified. Moreover, as Nadia Drake recounts in this absorbing look back at her father’s work in that era, Drake had spent the time immediately before the meeting trying to line up Champagne for UC-Berkeley biochemist Melvin Calvin, who was about to win the Nobel Prize.
So there was a certain ad hoc flavor to the equation, one that Drake assembled more or less on the fly to clarify the factors to be considered in looking for other civilizations. How Drake did all this while trying to locate a sufficient quantity of good Champagne in the rural West Virginia of 1961 is beyond me and adds to his mystique.
Image: Astronomer Frank Drake speaking at Cornell University in Schwartz Auditorium, 19 October 2017. Credit: Wikimedia Commons CC BY-SA 4.0.
Sparkling wine aside, the Drake Equation in various forms has continued to inform discussion. The likelihood of detecting alien civilizations could be approached by multiplying the seven factors Drake came up with, which are shown in the figure below. The number of detectable alien civilizations is N. The Drake Equation obviously relied on guesswork at the time, given that we knew little about the factors involved other than the rate of star formation.
Image: The Drake Equation. Credit: Ming Hsu (UC-Berkeley).
There’s still a lot of play in these numbers, of course, but it’s fascinating to watch the progress of exoplanetary science as we begin to fill in the numbers through actual observation. Notice in particular ne the number of planets, per star, that could support life. This value is what gets play in the recently released (on arXiv) paper from Steve Bryson and a large number of colleagues at the SETI Institute, NASA and a variety of other organizations.
What catches the eye is the figure of 300 million, which is the number the researchers give for potentially habitable planets in the Milky Way. Let’s drill into this a bit: The researchers are computing the occurrence of rocky worlds, defined here as planets within a certain range of radius (0.5 R? ? r ? 1.5 R?), orbiting stars with effective temperatures of 4,800-6,300 K. The host stars cover main-sequence dwarf stars from Kepler’s DR25 planet candidate catalog as well as stars in data compiled by the European Space Agency’s Gaia mission. As the authors note: “We base our occurrence rates on differential population models dependent on radius, instellation flux and host star effective temperature.”
This is a change of pace from the norm, so let’s turn to the paper:
Most of the existing literature on habitable zone occurrence rates are in terms of orbital period, where a single period range is adopted to represent the bounds of the habitable zone for the entire stellar population considered. However, no single period range covers the habitable zone for a wide variety of stars…While these period ranges cover much of the habitable zone for G stars, they miss significant portions of the habitable zones of K and F stars, and include regions outside the habitable zone even when restricted to G stars. This will be true for any fixed choice of orbital period range for the range of stellar effective temperatures required for good statistical analysis. Such coverage will not lead to accurate occurrence rates of planets in the habitable zone.
Hence the decision to work with instellation flux, which measures the photon flux on each planet as received from its host star. The authors say that this is the first paper on occurrence rates for habitable zone planets that operates on star-dependent photon output. In terms of effective temperature, G-class stars like the Sun are in the range of 5,200–6,000 K. F-class is 6,000–7,500 K, but as the paper notes, the paucity of F stars in the sampled data leads to the authors setting the temperature limits lower. K-class stars show up at effective temperatures of 3,700–5,200 K. The range used in this paper — 4,800-6,300 K — also excludes M-dwarfs, whose effective temperatures range from 2,400–3,700 K.
Leaving out M-dwarfs could substantially under-count habitable zone worlds, but we also have enough concerns about tidal lock, stellar flare activity and atmospheric loss that we can’t assume M-dwarf planets are habitable. In any case, the authors have other reasons for the decision, including a very practical matter of future observation. After all, an analysis like this may well be useful as we ponder our target lists, and we also have to remember the limits of transit observation Kepler had to deal with:
The reason for limiting to Teff > 4800 K is two fold: (1) The inner working angle (IWA, the smallest angle on the sky at which a direct imaging telescope can reach its designed ratio of planet to star flux) for the LUVOIR coronagraph instrument ECLIPS falls off below 48 milliarc sec at 1 micron (3?/D) for a planet at 10 pc for Teff ? 4800 K, and (2) Planets are likely tidal-locked or synchronously rotating below 4800 K that could potentially alter the inner HZ limit significantly…The upper limit of 6300 K is a result of planets in the HZs having longer orbital periods around early F-stars, where Kepler is not capable of detecting these planets…
So bear this in mind: Excluding what could be vast numbers of habitable planets in M-dwarf orbits, we still wind up with 300 million possibilities in the broad range of K-class through G-class stars. Co-author Jeff Coughlin is director of Kepler’s Science Office:
“This is the first time that all of the pieces have been put together to provide a reliable measurement of the number of potentially habitable planets in the galaxy. This is a key term of the Drake Equation, used to estimate the number of communicable civilizations — we’re one step closer on the long road to finding out if we’re alone in the cosmos.”
Image: This illustration depicts Kepler-186f, the first validated Earth-size planet to orbit a distant star in the habitable zone. Credit: NASA Ames/JPL-Caltech/T. Pyle.
When you go through this paper, bear in mind what Centauri Dreams associate editor Alex Tolley pointed out to me — The Drake ne factor refers to the number of planets per star that can support life. What the Bryson et al. paper takes as its starting point is the number of rocky planets in the habitable zone, and this could mean that the figure of 300 million ‘habitable’ worlds takes in planets that resemble Venus more than Earth. It may also include water worlds, where the likelihood of technological civilization is unknown.
So Drake’s term ne is not the same value as taken up in the new paper. Nonetheless, let’s return to that dazzling figure of 300 million, because when we’re dealing with that many planets of interest, we can afford to lose a number that turn out to be uninhabitable and still consider ourselves overwhelmed with possibilities for life.
Numbers like these have implications for stars relatively near the Sun. The authors look at both the conservative and optimistic habitable zone, with the narrower ‘conservative habitable zone’ bounded by the ‘moist greenhouse’ and ‘maximum greenhouse’ limits, and the wider ‘optimistic habitable zone’ bounded by the ‘current Venus’ and ‘early Mars’ limits. I’m drawing this descrtiption from the Planetary Habitability Laboratory’s summary of work by Ravi kumar Kopparapu and colleagues (citation below).
Image: Habitable Zone of around main sequence FGKM stars. The warm ‘habitable’ zone is divided into a ‘conservative habitable zone’ (light green) and an ‘optimistic habitable zone’ (dark green). Earth is at the inner edge of the ‘conservative habitable zone.’ Credit: PHL.
Filtering their results using calculations for the conservative habitable zone, the authors maintain they can say with 95 percent confidence that the nearest rocky habitable zone planet around either a G- or K-class star is within 6 parsecs (roughly 20 light years). There could be four habitable zone rocky planets around G- and K-dwarfs within 10 parsecs of the Sun.
How to build our small planet catalog to reduce uncertainties in the calculations? The answer is clearly more space-based observations even as new ground-based telescopes come online. Let’s also remember what we lost because of Kepler’s mechanical problems. While we did get a K2 extended mission, the original Kepler extended mission was meant to continue the ‘long stare’ at the original starfield, adding four more years of precision photometric data. The number of small planets in the habitable zone would have been significantly extended.
…by definition, Kepler planet candidates must have at least three observed transits. The longest orbital period with three transits that can be observed in the four years of Kepler data is 710 days (assuming fortuitous timing in when the transits occur). Given that the habitable zone of many F and late G stars require orbital periods longer than 710 days, Kepler is not capable of detecting all habitable-zone planets around these stars.
Given that upcoming missions like PLATO do not include such long stares on a single field of stars (PLATO plans no more than 3 years of continuous observation of a single field), we will need future missions to achieve what the original Kepler extended mission might have done, which would have been a doubling of the DR25 dataset and a large yield of small habitable zone planets.
The paper is Bryson et al., “The Occurrence of Rocky Habitable Zone Planets Around Solar-Like Stars from Kepler Data,” in process at the Astronomical Journal (abstract). The Kopparapu et al. paper is “Habitable Zones Around Main-Sequence Stars: New Estimates,” Astrophysical Journal Vol. 765, No. 2 (26 February 2013). Abstract.
A Fast Radio Burst in the Milky Way
A sequence of new observations gives us a leading candidate to explain Fast Radio Bursts (FRBs). These powerful bursts of radio waves, lasting but milliseconds, first turned up in our data in 2007 and have been a mystery ever since. As they were found in other galaxies, it has been difficult to determine their exact location, and they were impossible to predict as most seemed to be one-off events, although astronomers have subsequently found some that do repeat.
Among the possible causes of FRBs, stellar remnants have been put forward, with the kind of highly magnetic neutron stars called magnetars receiving close scrutiny because their magnetic fields could be the engine driving the bursts. We now have three papers in Nature that give us tight observational evidence of the kind that has been lacking. Between the three, we have data that for the first time link an FRB in our own galaxy to a magnetar, the object known as SGR 1935+2154, located in the constellation Vulpecula.
Image: Artist’s impression of a magnetar in outburst, showing complex magnetic field structure and beamed emission, here imagined as following a crust cracking episode. Credit: McGill University Graphic Design Team.
Let’s look at the events involved here. We begin with two space-based observatories. Both the Fermi Gamma-ray Space Telescope and the Neil Gehrels Swift Observatory registered rapid bursts of gamma- and X-rays from this object in late April. The spate of burst activity lasted for hours and was also observed by NASA’s Neutron star Interior Composition Explorer, an X-ray telescope mounted on the International Space Station.
This was followed by observations about thirteen hours later of another X-ray burst, once again seen by numerous instruments, including the European Space Agency’s INTEGRAL mission as well as China’s Huiyan X-ray satellite and the Russian Konus gamma-ray burst monitor, a Russian experiment that flies aboard NASA’s GGS-Wind spacecraft. This was a burst lasting about half a second, but as it flared, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and the Survey for Transient Astronomical Radio Emission 2 (STARE2), located at CalTech, detected a Fast Radio Burst (FRB 200428) in the same part of the sky.
Scientists at the Chinese Five-hundred-meter Aperture Spherical radio Telescope (FAST) had been observing SGR 1935+2154 for some weeks. As I understand it, they did not see FRB 200428, but were subsequently able to detect an FRB at the same location (FAST was not observing the object when FRB 200428 was detected by the other instruments).
Citations for papers from both the CHIME/FRB Collaboration and the STARE2 team are given below, along with a citation for a paper from FAST astronomers that provides limits to the radio flux of the FRB. The authors of the latter find a ‘weak correlation’ between FRBs and SGR 1935+2154, as co-author Zhang Bing (University of Nevada) explains:
“The weak correlation could be explained by special geometry and/or limited bandwidth of FRBs. The observations of SGR J1935 start to reveal the magnetar origin of FRBs, although other possibilities still exist.”
So we have a burst of X-rays associated with an FRB, the latter radio component detected first at CHIME and then STARE2. Moreover, the radio burst from SGR 1935+2154 was thousands of times brighter than any radio emissions previously observed from magnetars in the Milky Way, and in fact would have registered as a weak FRB had it occurred in another galaxy. The arrival of the radio pulse during an X-ray burst points to this magnetar as the source.
Paul Scholz is a researcher at the University of Toronto’s Dunlap Institute for Astronomy & Astrophysics and a member of the CHIME/FRB Collaboration:
“The radio burst was far brighter than anything we had seen before, so we immediately knew it was an exciting event. We’ve studied magnetars in our galaxy for decades, while FRBs are an extragalactic phenomenon whose origins have been a mystery. This event shows that the two phenomena are likely connected.”
Image: A powerful X-ray burst erupts from a magnetar – a super-magnetized version of a stellar remnant known as a neutron star – in this illustration. A radio burst detected April 28 occurred during a flare-up like this on a magnetar called SGR 1935. Credit: NASA’s Goddard Space Flight Center/Chris Smith (USRA).
The magnetic field of a magnetar can be a thousand times stronger than that of a typical neutron star, offering us a generous source of power, but at the same time, the radio burst from SGR 1935+2154 was thousands of times brighter than any radio emissions ever detected from magnetars in our galaxy. Paul Scholz has speculated that it may require the youngest, most active magnetars to explain the range of FRB sources we’ve thus far observed elsewhere, although we have yet to make a simultaneous detection tying an FRB with an X-ray burst in another galaxy, a signature that efforts like CHIME will continue to look for.
George Younes (George Washington University) is lead author on two upcoming papers on the FRB. Here he speculates on the nature of the burst activity:
“The bursts seen by NICER and Fermi during the storm are clearly different in their spectral characteristics from the one associated with the radio blast. We attribute this difference to the location of the X-ray flare on the star’s surface, with the FRB-associated burst likely occurring at or close to the magnetic pole. This may be key to understanding the origin of the exceptional radio signal.”
Image: This aerial view shows the Canadian Hydrogen Intensity Mapping Experiment (CHIME), a radio telescope located at Dominion Radio Astrophysical Observatory in British Columbia. Credit: Richard Shaw/UBC/CHIME Collaboration.
So we have the first FRB found in the Milky Way, one whose association with a magnetar may help us place these bursts in context now that we know magnetars can drive FRB activity. On exactly how a magnetar produces an FRB, I want to quote Amanda Weltman and Anthony Walters, who provide a fine overview of this work in an article in Nature that is not behind the journal’s firewall:
…there are several mechanisms by which magnetars can drive FRBs, each of which has a distinct observational signature. The new results thus open up a host of exciting problems to explore. For example, what theoretical mechanism could give rise to such bright, yet rare, radio bursts with X-ray counterparts? One promising possibility is that a flare from a magnetar collides with the surrounding medium and thereby generates a shock wave. Observations of nearby rapidly star-forming galaxies will be crucial for finding events similar to FRB 200428, to help pin down the actual mechanism.
The paper from the STARE team is Bochenek et al., “A fast radio burst associated with a Galactic magnetar,” Nature 587 (2020), pp. 59-62 (abstract). The CHIME/FRB paper is “A bright millisecond-duration radio burst from a Galactic magnetar,” Nature 587 (2020), pp. 54-58 (abstract). The Lin paper is “No pulsed radio emission during a bursting phase of a Galactic magnetar,” Nature 587 (2020), pp. 63-65 (abstract).
Voyager 2: Back in Two-Way Communication
It’s reassuring to hear that we’re in two-way contact once again with Voyager 2. Since last March, controllers have been limited to receiving X-band (8 to 12 GHz) downlink data, with no capability to uplink commands to the craft via S-band (2 to 4 GHz). This has been a problem unique to Voyager 2 thanks to its trajectory. The Deep Space Network’s three radio antenna facilities — Canberra, Australia; Goldstone, California and Madrid, Spain — are positioned so that at least one facility is available for communications with our far-flung space probes.
While Voyager 1 can talk to us via the two northern hemisphere DSN stations, Voyager 2’s close flyby of Neptune’s large moon Triton in 1989 bent its course well south of the ecliptic. 18.8 billion kilometers from Earth, Voyager 2 can only line up on Canberra, and the antenna called Deep Space Station 43 (DSS43) has been the only southern hemisphere dish with a transmitter capable of reaching the craft at the right frequency to send commands. DSS43 went offline for equipment upgrades to handle increasingly problematic aging equipment.
Philip Baldwin is operations manager for NASA’s Space Communications and Navigation (SCaN) Program;
“The DSS43 antenna is a highly specialized system; there are only two other similar antennas in the world, so having the antenna down…is not an ideal situation for Voyager or for many other NASA missions. The agency made the decision to conduct these upgrades to ensure that the antenna can continue to be used for current and future missions. For an antenna that is almost 50 years old, it’s better to be proactive than reactive with critical maintenance.”
Image: Crews conduct critical upgrades and repairs to the 70-meter-wide radio antenna Deep Space Station 43 in Canberra, Australia. In this image, one of the antenna’s white feed cones (which house portions of the antenna receivers) is being moved by a crane. Credit: CSIRO.
Only DSS43 has the S-band transmitter powerful enough to reach and communicate with Voyager’s dated technology, but the upgrades will also be significant for Moon and Mars missions going forward. From the Voyager 2 perspective, the craft has been in a quiescent mode that still allowed return of science data, with Canberra’s three 34-meter dishes configured to listen to its signal, though unable to transmit commands. Now DSS43 has successfully tested its new hardware and we can get back to uploading commands as needed to the probe.
“What makes this task unique is that we’re doing work at all levels of the antenna, from the pedestal at ground level all the way up to the feedcones at the center of the dish that extend above the rim,” said Brad Arnold, the DSN project manager at NASA’s Jet Propulsion Lab in southern California. “This test communication with Voyager 2 definitely tells us that things are on track with the work we’re doing.”
All of which is excellent news, as Voyager 2 seems healthy. Recall that the spacecraft accidentally overdrew its power supply last January, leading to the automatic shutdown of its science instruments. The upgrade of DSS43 began after that problem had been resolved. The station now has two new radio transmitters as well as upgraded heating and cooling equipment, power supply and support upgrades demanded by the new transmitters.
So both our active craft in interstellar space are in two-way communication again. It’s instructive, and a bit awe-inspiring, to remember how difficult it is to track a signal as weak as the one Voyager can produce. At Jupiter, the craft could transmit at 115,000 kilobits per second. This is a 23-watt radio transmitter that produced, at the giant planet, a signal that was one hundred-millionth as powerful as a cell phone battery by the time it reached the DSN on Earth. And of course Voyager keeps going. Even before it went interstellar, Voyager 2’s power levels received at Earth were more than five hundred times fainter than at the Jupiter encounter.
We may be able to keep the Voyagers alive into the middle of this decade by clever cycling of their instruments and systems, with the last surviving science instrument likely being the magnetometer, which has the lowest power requirement. A faint engineering signal might still be feasible into the 2030s, but no one knows exactly. It would be wonderful if we could stay in touch with these craft until 2027, which would mark their 50th year in flight. Go Voyager.
Speculations on Starless Worlds
Yesterday’s paper from Matt Clement and team reminded us of the enormous transformation that can take place in a planetary system as it lurches toward eventual stability. Gas giants have so much to say about how this process occurs, with their gravitational interactions sometimes ejecting other worlds from the system. Ejected planets are often called ‘rogue’ planets because they wander the galaxy without orbiting a star. Their numbers may be vast.
Clement and team think we may have ejected an ice giant from our early system, as we discussed yesterday. Whatever the case, I’ve been talking about rogue planets for about ten years, and as I look back, I run into intriguing finds like PSO J318.5-22, which is described in a 2013 paper from Michael Liu and colleagues (citation below). Says Liu (University of Hawaii):
“We have never before seen an object free-floating in space that looks like this. It has all the characteristics of young planets found around other stars, but it is drifting out there all alone.”
We learn from the paper that this object shows similarities to young, dusty planets in terms of luminosity, mass, spectrum, etc. It’s also useful because it’s likely a member of the ? Pic moving group, meaning we can glean something about its age, about 12 million years. Have a look.
Image: One apparent free-floating planet turned up in a search for brown dwarfs. This multicolor image is from the Pan-STARRS1 telescope, showing PSO J318.5-22, in the constellation of Capricorn. The planet is extremely cold and faint, with most of its energy emitted at infrared wavelengths. The image is 125 arcseconds on a side. Credit: N. Metcalfe & Pan-STARRS 1 Science Consortium.
Estimates of the number of rogue planets are all over the map, with one recent one being 2000 objects ranging in size between the Moon and Jupiter per main sequence star. That’s from a 2018 study by Xinyu Dai & Eduardo Guerras (University of Oklahoma), but if we want to jump to the high end, we can go with Louis Strigari (Stanford University) and colleagues: 105 compact objects per main sequence star. See Island Hopping to the Stars for Strigari, and Detection of Extragalactic Planets? for Dai and Guerras, where I give citations.
Now we have word of a small rogue world probably about the size of the Earth. Przemek Mroz (California Institute of Technology) is lead author of the study. Detected through gravitational microlensing, which is about the only way it could have been found with our current technologies, the world is labelled OGLE-2016-BLG-1928. This is the smallest rogue candidate yet identified, with the microlensing event having a timescale of a scant 42 minutes.
Image: An artist’s impression of a gravitational microlensing event by a free-floating planet. Credit: Jan Skowron / Astronomical Observatory, University of Warsaw.
Gravitational microlensing occurs when a foreground star (or planet, in this case) moves in front of a background stellar object, causing the light from the more distant star to be magnified. A brief burst in magnification becomes the signal identifying the star and any associated exoplanet, but in the case of rogue planets, we have no central star. Here’s Mroz on the matter:
“If a massive object (a star or a planet) passes between an Earth-based observer and a distant source star, its gravity may deflect and focus light from the source. The observer will measure a short brightening of the source star. Chances of observing microlensing are extremely slim because three objects – source, lens, and observer – must be nearly perfectly aligned. If we observed only one source star, we would have to wait almost a million year to see the source being microlensed.”
Image: Changes of brightness of the observed star during the gravitational microlensing event by a free-floating planet. Credit: Jan Skowron / Astronomical Observatory, University of Warsaw.
Given how few of these objects we have detected, and the wide range of estimates in the population of rogue planets, it’s hard to make too many statements about them. Are they all the likely result of gravitational interactions in an infant or maturing stellar system? Is there a risk of mistaking ultracool brown dwarfs for planets in this regime (I assume the answer is yes)? In the case of OGLE-2016-BLG-1928, are we absolutely sure there is no host star? Consider this passage from the paper:
The discovery of OGLE-2016-BLG-1928 demonstrates that current microlensing surveys are capable of finding extremely-short-timescale events. Although the mass of the lens cannot be unambiguously measured, properties of the event are consistent with the lens being a sub-Earth-mass object with no stellar companion up to the projected distance of ? 8 au from the planet. Thus, the lens is one of the best candidates for a terrestrial-mass rogue planet detected to date. This population of low-mass free-floating (or wide-orbit) planets may be further explored by the upcoming microlensing experiments.
Would OGLE have detected a star here if the planet in question was at Saturn’s 10 AU distance from it? This was one tricky detection, “at the edge of current limits of detecting short-timescale microlensing events,” according to the authors. It’s suggestive of a rogue planet. Looking ahead, we’re also moving toward a space-based microlensing capability in the Nancy Grace Roman Space Telescope (WFIRST). Future surveys will doubtless turn up rogue planets that will add to the ground-breaking work accomplished by dedicated ground-based surveys like OGLE.
And I wonder: What happened to that third ice giant that was ejected from our early Solar System? Is it out there wandering the galaxy, yet another rogue far from the star of its birth?
The paper is Mroz et al., “A terrestrial-mass rogue planet candidate detected in the shortest-timescale microlensing event,” Astrophysical Journal Letters Vol. 903, No. 1 (29 October 2020). Abstract / Preprint. The Liu paper is “The Extremely Red, Young L Dwarf PSO J318-22: A Free-Floating Planetary-Mass Analog to Directly Imaged Young Gas-Giant Planets,” Astrophysical Journal Letters Vol. 77, No. 2 (22 October 2013). Abstract.
Jupiter, Saturn and the Early Solar System
The days when scientists assumed our Solar System would be something of a template for planetary systems elsewhere are long past. The issue now is to delve deeper into system architectures to figure out what happens in their infancy and how they evolve. Working backward from today’s Solar System is one way to approach the problem. Thus Matt Clement (Carnegie Institution for Science), who has led a recent study into the formation of Jupiter and Saturn, hoping to determine how they wound up in their present orbits. Says Clement:
“We now know that there are thousands of planetary systems in our Milky Way galaxy alone. But it turns out that the arrangement of planets in our own Solar System is highly unusual, so we are using models to reverse engineer and replicate its formative processes. This is a bit like trying to figure out what happened in a car crash after the fact–how fast were the cars going, in what directions, and so on.”
Image: New work led by Carnegie’s Matt Clement reveals the likely original locations of Saturn and Jupiter. Credit: Saturn image is courtesy of NASA/JPL-Caltech/Space Science Institute.
As you would imagine, work like this involves numerous computer simulations, some 6,000 of which are described in the paper in Icarus. The Solar System we were born into emerges in these simulations from conditions that were markedly dissimilar in the early days of planet formation, when the larger bodies in the system began to spring gravitational surprises on a tidy early arrangement, with a re-shuffling that gave shape to today’s orbital stability.
Our models for how all this happened are now being tweaked, as exemplified by Clement and team’s work. Whereas Jupiter was once believed to orbit the Sun three times for every two orbits completed by Saturn — this has evolved within the ‘Nice Model’ of system formation — the new computer simulations show that the arrangement of planets we have today is better produced by starting with a ratio of two Jupiter orbits to one Saturn orbit.
As the authors put it, “adequately exciting Jupiter’s eccentricity without exceeding Jupiter and Saturn’s modern orbital spacing is extremely challenging.” The Nice Model (named after the city in France, where it was originally developed at the Observatoire de la Côte d’Azur) considers the migration of the giant planets after the dissipation of the early protoplanetary disk. Clement’s simulations show the effect of the Kuiper Belt on the positions of Uranus and Neptune, with signs of an early ice giant that was ejected during the roiling period of system formation.
Image: Jupiter in its infancy was thought to orbit the Sun three times for every two orbits that Saturn completed. But this arrangement is not able to satisfactorily explain the configuration of the giant planets that we see today. Matt Clement and his co-authors showed that a ratio of two Jupiter orbits to one Saturnian orbit more consistently produced results that look like our familiar planetary architecture. Credit: NASA.
In the Nice Model, the planets emerge from the protoplanetary disk in a compact, resonant configuration, with subsequent perturbations breaking the resonance of one or more planets, creating a phase of dynamical instability that reshapes the outer Solar System. The Nice Model is widely accepted in the field because of its ability to predict the orbital shape of the Kuiper Belt and main asteroid belt, and properties of some gas giant moons.
The Nice Model seems strong, and as the paper points out, its simulations replicate the capture of irregular moons in the outer Solar System and allow the survival of the asteroid belt while explaining the trojan asteroids. As the authors go on to say, “A major potential pitfall of the primordial 2:1 version of the Nice Model presented in this manuscript is its effects on the asteroid belt and terrestrial-forming regions.”
Hence the need to investigate the consequences of the proposed scenario on the inner Solar System. But the paper discusses the problems resolved by assuming an initial 2:1 resonance. The authors acknowledge that some aspects of the system’s present architecture are still low-probability outcomes. A major issue is eccentricity. Were the gas giants actually born in a 2:1 resonance and in orbits that were at origin somewhat eccentric?
That scenario, described as “somewhat of a paradigm shift,” does produce the modern Jupiter-Saturn orbits and is consistent with the position of Uranus and Neptune, with this addition: “…we show that Uranus and Neptune’s final orbits are determined by a combination of the mass in the primordial Kuiper belt and that of an ejected ice giant.” Adds Clement:
“This indicates that while our Solar System is a bit of an oddball, it wasn’t always the case. What’s more, now that we’ve established the effectiveness of this model, we can use it to help us look at the formation of the terrestrial planets, including our own, and to perhaps inform our ability to look for similar systems elsewhere that could have the potential to host life.”
As we tighten the Nice Model further, the paper describes the research path ahead:
Though our work shows that the primordial 2:1 Jupiter-Saturn resonance is a viable evolutionary path for the solar system, future work is still required to fully validate our presumed initial conditions, and robustly analyze the consequences of such a scenario on the solar system’s fragile populations of small bodies. In particular, follow-on investigation of the giant planets’ instability evolution with Jupiter and Saturn in a primordial 2:1 MMR with enhanced eccentricities must consider longer integration times (≲100 Myr), higher resolution disks (≲10,000 particles), and account for the dissipating gaseous nebula.
Clement presented these results at the American Astronomical Society’s Division for Planetary Sciences virtual meeting, which ended on the 30th of October.
The paper is Clement et al., “Born eccentric: Constraints on Jupiter and Saturn’s pre-instability orbits,” Icarus Vol. 355 (February 2021), 114122 (abstract).
Getting Ready for Dragonfly: Titan’s Impact Craters
What accounts for the differences in Titan’s craters? It will be helpful from an operational standpoint to learn more, for in 2027 the Dragonfly mission will launch, with Selk Crater a target. An equatorial dune crater, Selk is completely covered in a dark organic material, unlike other higher-latitude craters on the Saturnian moon that are scoured and cleansed by rain. We have learned from data produced by Cassini’s Visible and Infrared Mapping Spectrometer (VIMS) that Titan’s craters come in two kinds. Equatorial craters like Selk occur in dune fields and consist mostly of organics. Mid-latitude craters show a mix of organics and water ice.
The organic material generated by processes in Titan’s thick atmosphere is sand-like, piling up in equatorial regions but being eroded at the higher, wetter latitudes. For Dragonfly’s purposes, we want to know more about how the methane rain and streams affect the surface as we fine-tune the data analysis and monitoring techniques to be used in the mission.
What’s happening in Titan’s craters reminds us just how active the surface here is, says Anezina Solomonidou, a research fellow at the European Space Agency and lead author of a paper that explores the issue:
“The most exciting part of our results is that we found evidence of Titan’s dynamic surface hidden in the craters, which has allowed us to infer one of the most complete stories of Titan’s surface evolution scenario to date. Our analysis offers more evidence that Titan remains a dynamic world in the present day.”
Image: This composite image shows an infrared view of Saturn’s moon Titan from NASA’s Cassini spacecraft, acquired during the mission’s “T-114” flyby on Nov. 13, 2015. The spacecraft’s visual and infrared mapping spectrometer (VIMS) instrument made these observations, in which blue represents wavelengths centered at 1.3 microns, green represents 2.0 microns, and red represents 5.0 microns. A view at visible wavelengths (centered around 0.5 microns) would show only Titan’s hazy atmosphere. The near-infrared wavelengths in this image allow Cassini’s vision to penetrate the haze and reveal the moon’s surface. Credit: NASA/JPL/University of Arizona/University of Idaho.
Look closely at the above image and you’ll see Titan’s largest confirmed impact crater, called Menrva, near the limb above center to the left. Cassini was at about 10,000 kilometers from the moon during this approach, a good deal higher than many flybys, but the altitude allowed the VIMS instrument to cover wide areas at moderate resolution. You can also see two dark bands, parallel regions filled with dunes at the center of the image, with some regions of finer resolution inset in the image; these were acquired near the spacecraft’s closest approach.
But back to the evolution of those craters, which is the subject of the paper in Astronomy & Astrophysics. The differences between craters are telling, for they point to different evolution depending on geography. When objects make it through Titan’s atmosphere to impact on the surface, the heat generated by the event mixes organic materials and water ice from below. The cleansing methane rain subsequently falls in the mid-latitude plains, whereas in the equatorial regions, the impact areas are covered by a layer of sandy organic sediment.
The two classes of impact crater are strikingly different. Processes after the impact account for the outcomes. From the paper:
These observations agree with the evolution scenario proposed by Werynski et al. (2019), wherein the impact cratering process produces a mixture of organic material and water ice, which is later “cleaned” through fluvial erosion in the mid-latitude plains. However, the cleaning process does not appear to operate in the equatorial dunes; rather, the dune craters are quickly covered by a thin layer of sand sediment. This scenario agrees with other works that suggest that atmospheric deposition is similar in the low-latitudes and midlatitudes on Titan, but with more rain falling onto the higher latitudes causing additional processing of materials in those regions… In either case, it appears that active processes are working to shape the surface of Titan, and it remains a dynamic world in the present day.
Image: These six infrared images of Saturn’s moon Titan represent some of the clearest, most seamless-looking global views of the icy moon’s surface produced so far. The views were created using 13 years of data acquired by the Visual and Infrared Mapping Spectrometer (VIMS) instrument on board NASA’s Cassini spacecraft. Credit: NASA/JPL-Caltech/Stéphane Le Mouélic, University of Nantes, Virginia Pasek, University of Arizona.
With a sea of water and ammonia beneath the crust, Titan is a place where a large impact will move organic materials between the surface and the ocean below, highlighting the significance of Dragonfly’s future work at Selk Crater as a way to probe the moon’s composition. And while it seems likely that erosion has obscured most impact craters from Titan’s past, we do have 90 potential features, according to the paper, that may be craters to work with. These features offer a window into the atmosphere’s influence on the surface through weathering while exposing material from the interior which Cassini’s RADAR instrument was unable to probe.
The paper is Solomonidou et al., “The chemical composition of impact craters on Titan,” Astronomy & Astrophysics Vol. 641, A16 (September 2020). Abstract.
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