by Paul Gilster | Jun 11, 2020 | Outer Solar System |
Finding out that Titan is migrating away from Saturn should cause little surprise. Our own Moon moves away from the Earth at about 38 millimeters per year (even as Earth’s rotation slows ever so slightly, lengthening the day by 23 microseconds every year). Titan’s gravitational pull on Saturn causes frictional processes inside the giant world that ultimately impart energy to Titan, moving it away from its host in a similar way. The surprise attendant to a new paper on this phenomenon is the size of the movement, about 100 times greater than had been expected.
The paper explains the migration process like this:
Tidal friction within Saturn causes its moons to migrate outwards, driving them into orbital resonances that pump their eccentricities or inclinations, which in turn leads to tidal heating of the moons.
What we’re wrestling with here are the processes of energy dissipation in giant planets, which determine the timescale for their moons’ tidal migration. The theory advanced in this work may explain them.
The paper appears in Nature Astronomy, with Valéry Lainey (Paris Observatory) as lead author. Two teams of scientists used both astrometric and radiometric datasets, two different approaches into the same question, to measure Titan’s orbit over a ten year period. Astrometry produced measurements of Titan’s position in relation to background stars, tapping data from the Cassini orbiter. The radiometry work measured Cassini’s velocity as it was affected by the gravitational influence of Titan, revealed by analysis of the spacecraft’s radio transmissions during ten close flybys of the giant moon.
Usefully, the two datasets produced results in tight agreement. Co-author Jim Fuller (Caltech) proposed in 2016 that Titan’s migration rate would be considerably faster than predicted by standard tidal theories. Fuller’s idea was that Titan’s effect on Saturn, a gravitational squeeze at a particular frequency, would create strong oscillations, what the paper refers to as “inertial waves,” inside the planet. The process is a tidal forcing effect known as ‘resonance locking.’ Saturn’s oscillations, Fuller believed, would cause energy to be dissipated, allowing Titan to migrate outward at a faster rate only weakly sensitive to orbital distance.
The Cassini data from two different analyses confirm what Fuller has been saying. Indeed, whereas the prediction had earlier been that Titan would be migrating outward at 0.1 centimeters per year, the actual number is 11 centimeters per year. Resonance locking means that we can’t assume that moons like Titan form at the orbital distance at which we see them now. Instead, we see at Saturn a system that Fuller believes evolved far more dynamically.
Image: A giant of a moon appears before a giant of a planet undergoing seasonal changes in this natural color view of Titan and Saturn from NASA’s Cassini spacecraft. Titan, Saturn’s largest moon, measures 5,150 kilometers across and is larger than the planet Mercury. This mosaic combines six images — two each of red, green and blue spectral filters — to create this natural color view. The images were obtained with the Cassini spacecraft wide-angle camera on May 6, 2012, at a distance of approximately 778,000 kilometers)from Titan. Image scale is 29 miles (46 kilometers) per pixel on Titan. Credit: NASA/JPL-Caltech/Space Science Institute.
So we’re beginning to re-think how planets affect the orbits of their moons. These results suggest that Titan started out much closer to Saturn, with the system of moons expanding more quickly than earlier thought. Rather than assuming that outer moons like Titan migrated outward more slowly than inner moons, we learn that these outer moons can migrate at a similar rate. Migration turns out to be even more complex than first believed, with results at Saturn that have implications for how we study far more distant systems including exoplanets around host stars. Thus the paper’s conclusion:
Resonance locking could operate in other moon systems, such as the Jovian system, where it might drive the outward migration of Io/Europa/Ganymede and predicts a much smaller effective Q [the tidal quality factor] for Callisto if it is caught in a resonance lock. Resonance locking can also act in stellar binaries and exoplanetary systems, but it will not always dominate tidal dynamics, for instance, at very close separations when equilibrium tidal dissipation is more important, or when resonances are saturated by chaotic or non-linear effects. But resonance locking could be especially important at wider separations where equilibrium tidal dissipation is negligible (as it is for Titan’s migration), or in situations when a star or planet evolves on a relatively short timescale owing to a rapid evolutionary phase, accretion, magnetic braking or gravitational wave-driven inspiral.
The paper is Lainey et al., “Resonance locking in giant planets indicated by the rapid orbital expansion of Titan,” Nature Astronomy 8 June, 2020 (abstract).
by Paul Gilster | Jun 9, 2020 | Exoplanetary Science |
The planet candidate KOI-456.04 strikes me as significant not so much because of the similarity of its orbit with that of Earth (a 378 day orbital period around a star much like the Sun), but because of the methods used to identify its possible presence. Make no mistake, this is still very much a planet candidate, as co-authors René Heller and Michael Hippke are at pains to explain, noting that systematic measurement errors cannot be ruled out, though they estimate an 85 percent likelihood that it is there.
We don’t have many examples of small planets potentially in the habitable zone of a star like ours, and this is what has received the most media attention. So let’s look at this aspect of the story quickly, because I want to move past it. If this candidate is confirmed, it looks to be less than twice the radius of the Earth, receiving about 93 percent of Earth’s insolation from its star. Make assumptions about its atmosphere and you can arrive at a surface temperature averaging 5?, 10 degrees lower than Earth’s mean temperature.
Image: Most of the exoplanets from the Kepler mission are the size of Neptune and in relatively close orbits around their host stars, where temperatures on these planets would be far too hot for liquid surface water (third panel from above). Almost all of the Earth-sized planets known to have potentially Earth-like surface temperatures are in orbit around red dwarf stars, which do not emit visible light but infrared radiation instead (bottom panel). The Earth is in the right distance from the Sun to have surface temperatures required for the existence of liquid water. The newly discovered planet candidate KOI-456.04 and its star Kepler-160 (second panel from above) have similarities to Earth and Sun (top panel). MPS / René Heller.
But what I want to dwell on is the methodology used to study this system. Heller (Max Planck Institute for Solar System Research) and Hippke (Sonneberg Observatory, Germany) are joined here by colleagues at the University of Göttingen, UC-Santa Cruz and NASA Ames in a new look at archival data from Kepler on the star Kepler-160 in Lyra, which was observed by the mission between 2009 and 2013. The star is similar to the Sun in mass and radius and previously known to have two confirmed planets.
The new work analyzes transit timing variations in the orbital period of the planet Kepler-160c suggestive of a third planet. They find Kepler-160d, a third world that is disturbing the orbit of Kepler-160c. This is a planet without any transits that is thus only indirectly confirmed.
The intriguing candidate, potentially the fourth planet here, is KOI-456.04, which appears to be 1.9 Earth radii in an orbital period of 378 days. The Max Planck Institute for Solar System Research (MPS) happens to be building the PLATO Data Center, and the suggestion is that the PLATO mission, to be launched in 2026, will have the chance to confirm this interesting object of interest and study it in much greater detail.
Heller and Hippke have been developing their exoplanet detection pipeline in several recent papers, studying twelve detrending algorithms for stellar light curves in detail. ‘Detrending’ refers to eliminating noise within transit data to cull out evidence for a planet. The results pointed to a detrending algorithm available in the open source package called W?tan, used in combination with a transit search algorithm known as ‘transit least-squares’ as the most accurate choice. Heller and Hippke developed TLS specifically to look for smaller planets by modeling stellar limb darkening (see Dataset Mining Reveals New Planets for more on this).
What emerges is a more precise model of the brightness variations seen in a transit event, one that the duo believe improves upon the more established ‘box-like’ approximation known as the ‘box-fitting least square’ (BLS) algorithm. The latter is somewhat faster in computational terms, but the Wotan/TLS combination is in the authors’ view more sensitive. I talked to both Heller and Hippke about the new paper via email and asked Hippke about the advantages of their method. His response:
I… believe that W?tan+tls are the leading toolset in finding new transiting exoplanets. You gain about 10% sensitivity going from BLS to TLS. In other words, at the same false alarm rate (e.g., 1%) you get 10% more planets from TLS. Naturally these are at the small end of the size distribution (you find large planets as easily with BLS). Smaller planets are usually more interesting because rocky planets are believed to be < ~ 2 Earth radii.
The dominating noise source in transit observations is in many cases stellar variability, which is why Heller and Hippke tested a dozen detrending methods, all of which are available through W?tan (TLS is likewise an open source tool). According to Hippke, the W?tan methods are more important for more active stars — remember that M-dwarfs can be quite active in comparison to G-class stars like the Sun. Young stars just at the end of planet formation are likewise active, making them interesting targets for using the W?tan tools to achieve optimal detrending for exoplanet detection.
Heller told me that the team is 100% sure that Kepler-160d exists — this is the non-transiting world found through using transit timing variations of Kepler-160c. But what of the planet in the orbit roughly similar to that of the Earth around the Sun?
Our statistical analysis gives us 85% confidence that the signal belongs to a transiting planet. But 99% would be needed to call this a planet. In this case, this object would be called Kepler-160e. For now, it is not. So this object is transiting (I mean, if it is real in the first place), but we are less certain than for the non-transiting planet Kepler-160d that it actually exists. And so KOI-456.04 remains a candidate unless someone can show that it exists with more than 99% certainty.
Thus the tantalizing ‘world’ in the Earth-like orbit remains a Kepler Object of Interest (KOI), an object that cannot be currently validated or falsified, but one that will doubtless be on the target list for the PLATO exoplanet mission. The larger story is that the tools Heller and Hippke have deployed show the promise of pulling 10% more planets (and smaller ones at that) out of the raw data, which makes analysis of ongoing observations as well as reanalysis of older datasets more accurate. It will be fascinating to watch as the computational methods on display in this paper are applied to other known exoplanet systems, with their validity then put to the test by future space- and ground-based observatories.
The paper is Heller et al., “Transit least-squares survey. III. A 1.9 R? transit candidate in the habitable zone of Kepler-160 and a nontransiting planet characterized by transit-timing variations,” Astronomy & Astrophysics, Volume 638, id. A10 (June, 2020). Abstract/preprint.
by Paul Gilster | Jun 5, 2020 | Exoplanetary Science |
Selection is going to be a key issue for future ground- and space-based observatories. Given lengthy observing times for targets of high interest, we have to know how to cull from our exoplanet catalog those specific worlds that can tell us the most about life in the universe. Recently, Ramses Ramirez (Earth-Life Science Institute, Tokyo Institute of Technology) went to work on the question of habitable zones for complex life, which are narrower than the classic habitable zone defined by the potential for water on the surface. In today’s essay, Alex Tolley looks at Ramirez’ recent paper, which examines the question in relation to the solubility of gases in lipid membranes. What emerges in this work is a constrained habitable zone suited to complex life, with limits Alex explores. The model has interesting ramifications right here in the Solar System, but it also points the way toward constraining the list of planets upon which we’ll apply our emerging tools for atmospheric characterization.
By Alex Tolley
Daggerwrist on Darwin IV. Artist Wayne Douglas Barlowe. Source: Expedition.
Life on Earth, until its last three quarter-billion years, was almost entirely represented by unicellular organisms. As we explored in Detecting Early Life on Exoplanets, biosignatures for microbial life are likely to be far more prevalent than for worlds with complex life. While rocky worlds in the classic habitable zone (HZ) are still relatively few, academic PR departments trumpet every find as “Earth-like”, and a selection of these worlds will be targeted for biosignatures. However, as the number of these worlds increases, scientists will want to distinguish worlds that have a biosphere that can be characterized as more Earth-like, with verdant landscapes and megafauna in the seas and on land.
When the term “Earth-like” is used, the public thinks of a world that looks like Earth, with oceans, continents variously clothed in verdant landscapes, and perhaps most importantly of all, “charismatic megafauna”, the animals that you went to see at the zoo, or watched on David Attenborough’s excellent nature programs. A blue sea lapping on a muddy beach, despite teeming with microbes and other unicellular life, looks dead to the unpracticed eye, which means most of the human population. It is those human-scale animals like the daggerwrist pictured above from Barlowe’s “Expedition: Being an Account in Words and Artwork of the 2358 A.D. Voyage to Darwin IV” that excites the public.
If life is rare, then the classic HZ will have the least constraints, although most of those worlds will still have biospheres populated only with microbes, and fewer probably with unicellular plants and animals. If life is not rare, then there will be a desire to discover true Earth-like worlds with complex life, which may mean limiting the range of the HZ that will allow for such life to flourish.
The classic HZ range is defined by the possibility of liquid water remaining continuously on the surface, warmed by the star’s radiation and an atmosphere of sufficient pressure and with some greenhouse gases. This is because all Earth’s life requires liquid water and this has led to the mantra “Follow the water” for missions in the search for life. Inside the inner HZ limit, there will be a runaway greenhouse that eventually desiccates the planet, like Venus. Towards the outer edge, the atmosphere needs to be increasingly composed of greenhouse gases, particularly carbon dioxide (CO2) until a limit is reached.
For the solar system, the classic HZ lies at about 0.95 AU, inside Earth’s orbit, but excludes Venus, and extends to about 1.67 AU, outside of Mars’ orbit. It is this that offers the possibility of a second genesis and possibility of finding extant life in refuges and in the lithosphere beneath the now inhospitable Martian surface.
Complex, or multicellular, life on Earth emerged less than 1 billion years ago as photosynthesis reduced the CO2 in the atmosphere and replaced it with oxygen (O2). Except for a few recently discovered species, all multicellular life is aerobic and requires a rich O2 atmosphere. It is the much greater energy released by aerobic respiration compared to anaerobic respiration that allows for the energetic lifestyles of multicellular animal life (metazoa). At least for our planet, we believe that the conditions for complex life to survive are constrained; Earth has its own habitable zone limits that are narrower than the classic HZ. The question is, “What might those HZ limits be for complex life, and how does that translate for exoplanets around different stellar types?”
CO2 is one of the main greenhouse gases that extend the outer boundary of the HZ. Nitrogen (N2) also helps extend the outer edge of the HZ although it is not a greenhouse gas but a main constituent of the atmosphere. Are there limits to the pressures of these gases due to effects on complex life that limit the range of the possible HZ for multicellular life living on the planet’s surface?
A new paper by Dr. Ramses Ramirez attempts to answer that question by applying the relationship between the solubility of gases in lipid membranes and their anesthetic potency (see figure 1 below). This theory, a partial explanation for the still imperfectly understood mechanism of anesthesia, is that the solubility of gases in lipid membranes is correlated with their anesthetic potency. Anesthetists must monitor the use of these gases to maintain unconsciousness. Too little and the patient remains conscious of the pain during surgery, too much anesthetic, and the patient stops breathing and dies.
The anesthetic gases are to the bottom right of the chart in figure 1. Nitrous oxide (N2O) is less potent and still used in dentistry (as well as at “nitrous parties”). Less well known is that CO2 also acts as such a gas with solubility similar to N2O. Although physiologically CO2 initially increases breathing rate to flush it out of the lungs, at higher concentrations it then invokes respiratory, and later metabolic, acidosis, which sets in as CO2 dissolved in the blood serum eventually causes cessation of respiration and death. As can be seen in figure 1 below, N2 has low solubility in lipid membranes, 2 – 3 orders of magnitude lower than CO2, and concomitantly similar orders of magnitude lower anesthetic potency.
However, we are probably also familiar with the effects of high-pressure N2 as nitrogen narcosis that is experienced by divers breathing compressed air at depth. The argument is that both CO2 and N2 dissolving in the lipid membranes of cells will cause death if those gas concentrations reach the anesthetic level for complex life.
Figure 1: The Meyer-Overton correlation of oil/gas solubility versus anesthetic potential of inhaled gases. Figure recreated from published data. Source Ramirez [1].
Figure 1 above shows the relationship between gases and their anesthetic potential. CO2 solubility is similar to nitrous oxide, while N2 is far less potent and therefore apparently less of a constraint. Note that helium is at the upper end of the range and has low solubility and low anesthetic potency. This is why helium is used to replace N2 when deep diving in soft suits.
While the Meyer-Overton correlation is primarily for humans, it has been shown to apply across several different phyla as it is a physical, rather than physiologic effect. Determining the tolerance limits for CO2 and N2 provides a constraint that limits the HZ to a “Complex Life Habitable Zone (CLHZ).” Dr. Ramirez supports the general applicability of the lipid gas solubility to metazoa from prior experimental work, primarily on mammals, but also with other animals, to suggest that 0.1 bar (1/10th of surface atmospheric pressure or 1.4 psi) of CO2 might be a reasonable, conservative limit for complex life to tolerate CO2. N2 limits are primarily set by experiments for human divers. 2 bar of N2 seems to be the safe limit at which divers do not get narcosis. This is just 10 meters below the surface, a depth even beginner scuba divers can safely operate for short durations. Using upper limits for 0.1 bar CO2 and 2 bar N2, Dr. Ramirez finds that his radiative-convective model (RC) gives an estimated HZ for complex life (CLHZ) of 0.95 – 1.21 AU. Using an advanced energy balance model (EBM) that allows for different temperatures on the Earth’s surface, thus allowing for liquid water at the equator, but not at the poles, this CLHZ is extended from 0.95 – 1.31 AU.
The new outer range for this 2 bar N2 and 0.1 bar CO2 is 1.36 AU using the Energy Balance Model (EBM). This range is shown in figure 2 below not just for Earth, but for a range of main sequence star types. The relative decrease in the CLHZ compared to the HZ is greatest for cooler stars, the type we have most exoplanet examples in the HZ currently.
Figure 2. The Complex Life Habitable Zone (CLHZ) for A – M stars (2,600 – 9,000 K) compared to other definitions.The CLHZ is for a 0.1 bar, 2 bar N2 atmosphere which is compared to the classic HZ. While the inner edge of the HZ and CLHZ are the same at 0.95 AU, the outer edge of the CLHZ is now well inside the orbit of Mars. Image source: Ramirez.
Dr. Ramirez compares his results to a similar paper by Dr, Edward Schwieterman that looks at the same problem but through the lens of CO2 chemistry, with the note that carbon monoxide (CO), while not limiting the CLHZ, is toxic and could be limiting to the evolution of complex life [2]. (The CO is created by photolysis of CO2.) Schwieterman uses a 1D radiative-convective climate model for his calculations across a range of CO2 levels. Schwieterman does not investigate higher N2 pressures which results in his modeling having a narrower CLHZ than Dr. Ramirez’s most comparable modeling. However, the CO toxicity does not appear significant except for planets orbiting cool stars such as M dwarfs.
While both authors attempt to redefine the likely boundaries for the HZ of complex life based on Earth’s biological evolution, only Dr. Ramirez employs the possibility of increasing the N2 pressure to increase the outer limit.
To quote from the paper:
“The CLHZ is slightly wider at the higher N2 pressure because of increased N2-N2 collision induced absorption and a decrease in the outgoing infrared flux, which more than offset an increase in planetary albedo.”
Dr. Ramirez also states:
“I consider how our solar system’s HZ changes if we assume (for the moment) that complex life could evolve to breathe in a hypothetical 5-bar N2 atmosphere. For this sensitivity study, the RC model predicts that such worlds in our solar system can remain habitable at 1.24AU (SEFF = 0.65) whereas atmospheric collapse can be avoided as far as 1.36 AU (SEFF = 0.54) in the EBM (nearly 60% classical HZ width). I find that the additional N2 opacity is sufficient to counter the ice-albedo feedback, allowing for effective planetary heat transfer even at relatively far distances.”
Dr. Ramirez’s 0.1 bar constraint for CO2 should be put in context for life on Earth. CO2 is currently at about 0.04% (0.0056 psi) of the Earth’s atmosphere. Even during the Cambrian period when multicellular animals were rapidly diversifying into phyla, the atmospheric component of CO2 was never more than 1% and it fell fairly continuously during this period. The Great Permian Extinction which saw 90-95% of all complex life become extinct primarily by anoxia in the oceans, the CO2 levels were little more than 0.1% at their peak. [See “Climate Change and Mass Extinctions: Implications for Exoplanet Life”] and figure 3 below. For highly cognitive humans, NASA conservatively stipulated that the highest emergency level of CO2 in the Apollo Command and Lunar modules should be no more than 0.29 psi (0.02 bar) in an atmosphere of 5 psi O2 before cognitive skills become impaired [40]. The Centers for Disease Control and Prevention (CDC) guidelines for CO2 is that 0.04 bar CO2 is immediately dangerous [i].
It should also be noted that the analysis is limited to surface living, air-breathing animals. Bathypelagic organisms, such as oceanic fish may be adapted to tolerate far higher N2 pressures.
Figure 3. O2 and CO2 levels in the Phanerozoic. [3] While the Permian extinction is associated with a rise in CO2 levels to about 0.1%, and a decline in O2 levels from the Carboniferous, the CO2 levels were far higher at 1% at the start of the Cambrian and still high in the Devonian (the age of fishes).
But what about multicellular organisms other than animals? While Dr. Ramirez acknowledges that complex life includes plants and fungi, not just metazoa (animals), he is unable to address the possible range of CO2 and N2 pressures these complex life forms might be adapted to because there is next to no data on the effect high pressures and concentrations these gases have on plants or fungi, beyond incremental increases in CO2 to experiment on plant photosynthesis limits and productivity. Where we do have data is Earth’s history of complex life that indicates that relatively low levels of CO2 in the atmosphere due to volcanic emissions, and reduced plant life to draw down CO2 and replenish the O2 due to sulfur acid rains and ash-darkened skies, are sufficient to force most species, including plants, to extinction. We do not know what factor or combination of factors is important, nor whether it is primary factors such as anoxia, or n-th order factors that resulted in their final extinction.
Now that the inventory of exoplanets is rapidly increasing, it is certainly time that we start thinking more critically about what sort of life we are looking for and what that might mean for the range of the habitable zone that supports these different life forms. Rather than allowing the widest possible HZ that allows any atmospheric composition and pressure allowing liquid water, we could also be looking for possible constraints that appear required for the sort of surface, air-breathing complex life that will give rise to the charismatic fauna that we have on Earth. Dr. Ramirez has posited one interesting idea for terrestrial complex life that is based on respiration across a range of metazoans which then constrains the atmospheric gas composition and hence the HZ.
As Ramirez’ CLHZ has an outer limit well inside the orbit of Mars, this invites speculation that if Mars ever had any life during its earlier, wetter, period, it did not have complex life. If this model proves correct, while we may find subterranean microbial life on Mars, we will not find metazoan fossils, such as mollusk shells or vertebrate skeletons.
It should be borne in mind that life as a whole maintains Earth’s low CO2 levels to keep the surface temperature equitable for itself, maximizing biodiversity and biomass. While hotter (e.g. the Eocene maximum) and cooler (ice ages) periods upset that equitable temperature, life in concert with much slower geological processes act as a thermostat. It is also the case that biomass and diversity are greatest in the tropical forests and the lowest at the poles. It must have been relatively sparse during the “snowball Earth” period but recovered once the global ice sheets melted. Life has evolved on the Earth as it is, and has biochemistry that matches that requirement.
Today, that requirement is for an atmosphere that has a low CO2 level. On exoplanets, where much higher CO2 levels are needed to keep the planet warm, different biochemistries might develop, and this is a caveat that Ramirez considers for his analysis. However, without examples of such life, we are forced to use Earth’s life as our only model. In a half-billion or so years in the future, as the sun increases its luminosity, the required CO2 level to keep Earth cool enough will be below that needed by plants. A technological species might utilize technology like orbital sunshades or perhaps genetic engineering to maintain life on Earth.
The more important point is that we may be able to provide more granular characterizations of exoplanets. Rather than the binary in or out of the classic HZ for exoplanets and therefore potentially living or not, we can add granularity, such as inside the CLHZ and therefore capable of hosting complex life too. This conclusion does depend on exo-life following our terrestrial biology. If it doesn’t then we have to fall back to the more generous HZ calculations alone.
References
1. Ramirez, Ramses M. “A Complex Life Habitable Zone Based On Lipid Solubility Theory.” Scientific Reports, vol. 10, no. 1, 2020, doi:10.1038/s41598-020-64436-z.
2. Schwieterman, Edward W., et al. “A Limited Habitable Zone for Complex Life.” The Astrophysical Journal, vol. 878, no. 1, 2019, p. 19., doi:10.3847/1538-4357/ab1d52.
3. CO2 and O2 levels in the phanerozoic. Web accessed May 11, 2020. https://notrickszone.com/2018/05/28/2-new-papers-permian-mass-extinction-coincided-with-global-cooling-falling-sea-levels-and-low-co2/
4. Michel, E. L., et al, SP-368 Biomedical Results of Apollo – Chap. 5 Environmental Factors. Accessed from web, May 11th, 2020. https://history.nasa.gov/SP-368/s2ch5.htm
by Paul Gilster | Jun 4, 2020 | Exoplanetary Science |
55 Cancri e is a confirmed planet, and thus a departure from our topic of the last two days, which was the act of exoplanet confirmation as regards Proxima Centauri b and c, the latter still in need of further work before it can be considered confirmed. But 55 Cancri e has its uses in offering a tight orbit around a Sun-like star that can be detected using the transit method. That was just what was needed for ASTERIA (Arcsecond Space Telescope Enabling Research in Astrophysics), a technology demonstration mission involving a tiny CubeSat.
Sara Seager (MIT) has been at the heart of the investigation of CubeSats as exoplanet research platforms. I think the idea is brilliant. If we want to mount the most effective search of nearby Sun-like stars for Earth analogs, multiple telescopes must be in use. CubeSats are cheap. Why not launch a fleet of them, each with the task of monitoring a single star at a time. Launched in 2017, ASTERIA was the prototype, a nanosatellite equipped with precision pointing control and thermal stability of the sort needed to meet the tight tolerances of such observations.
Image: Left to right: Electrical Test Engineer Esha Murty and Integration and Test Lead Cody Colley prepare the ASTERIA spacecraft for mass-properties measurements in April 2017 prior to spacecraft delivery ahead of launch. ASTERIA was deployed from the International Space Station in November 2017. Credit: NASA/JPL-Caltech.
ASTERIA is a collaboration between the Jet Propulsion Laboratory and MIT, one in which MIT retains the lead in science operations while JPL handles overall project management. Seager is principal investigator on the project. Three mission extensions pushed the original 90 day prime mission into extensive prototype testing, which culminated in the CubeSat using its fine pointing control to detect 55 Cancri e’s transits. This is quite an achievement for the tiny satellite, given the need for a steady platform without movement or vibration as the star is examined.
And ponder this: 55 Cancri e blocks only 0.04% of the host star’s light. Mary Knapp is ASTERIA project scientist at MIT’s Haystack Observatory and lead author of the paper on this work, which will appear in the Astronomical Journal:
“We went after a hard target with a small telescope that was not even optimized to make science detections – and we got it, even if just barely. I think this paper validates the concept that motivated the ASTERIA mission: that small spacecraft can contribute something to astrophysics and astronomy.”
Image: The super-Earth exoplanet 55 Cancri e, depicted with its star in this artist’s concept, likely has an atmosphere thicker than Earth’s but with ingredients that could be similar to those of Earth’s atmosphere. Scientists say the planet may be entirely covered in lava. The planet is so close to its star that one face of the planet consistently faces the star, resulting in a dayside and a nightside. Credit: NASA/JPL-Caltech.
Yesterday we saw how data from three different sources were used to investigate Proxima Centauri c, strengthening the case for its existence but not yet confirming it. On its own, the ASTERIA data would be suggestive of a planet but not proof of it, but it was when comparing the CubeSat data with previous observations that it could be determined that the CubeSat had indeed seen the planet. As we develop CubeSat capabilities, we can use them to follow up on detections made by larger telescopes, focusing on one star and keeping our gaze fixed.
That’s especially useful for potential Earth analogs, where around G-class stars orbital times are long enough to require persistence if we are to see a transit. Thus it’s good news that Sara Seager has been awarded a NASA Astrophysics Science SmallSat Studies grant to develop a follow-on mission involving a constellation of satellites, each about twice the size of ASTERIA.
This excerpt from the paper describes the constellation concept:
ASTERIA was a successful technology demonstration of a future constellation of up to dozens of satellites, dubbed the ExoplanetSat Constellation. Each satellite would share ASTERIA’s precision pointing and thermal control capabilities, operate independently from the others, but may have different aperture sizes in order to reach down to fainter stars than ASTERIA’s current capability. The primary motivation is the fact that if there is a transiting Earth size planet in an Earth-like orbit about the nearest, brightest (V<7) Sun-like stars, we currently have no way to discover them; current missions saturate on these bright stars. The ultimate goal for the constellation is to monitor dozens of the brightest sun-like stars, searching for transiting Earth-size planets in Earth-like (i.e., up to one year) orbits.
The advantages of this ‘fleet’ approach are apparent. The paper continues:
Because the brightest sun-like stars are spread all across the sky, a single telescope will not do. Instead, each satellite would monitor a single sun-like star target of interest for as long as possible, before switching to another star, with targets only limited by the Sun, Earth and Moon constraints. To narrow down the approximately 3,000 target stars brighter than V=7, one would have to find a way to constrain the stellar inclinations and assume the planets orbit within about 10 degrees of the stars equatorial plane. This would reduce the number of target stars from about 3000 to about 300 (Beatty & Seager 2010), a much more tractable number of targets. The ExoplanetSat Constellation has a unique niche in context of existing and planned space transit surveys…, but is still in concept phase.
How to keep these spacecraft small? The use of CMOS detectors (complementary metal-oxide-semiconductor) working in visible light allowed ASTERIA to operate without a large cooling system, as would have been required by a CCD (charge-coupled device) to keep the instrument cold. We’ll follow MIT’s CubeSat work as the lessons learned from ASTERIA are drawn into the next design.
The paper is Knapp et al., “Demonstrating high-precision photometry with a CubeSat: ASTERIA observations of 55 Cancri e,” in process at the Astronomical Journal (preprint). Thanks to Centauri Dreams regular Andrew Tribick for the heads-up on this paper.
by Paul Gilster | Jun 3, 2020 | Exoplanetary Science |
Hard on the heels of the confirmation of Proxima Centauri b, we get news of Proxima c, which has now been analyzed in new work by Fritz Benedict (McDonald Observatory, University of Texas at Austin). Benedict has presented his findings at the ongoing virtual meeting of the American Astronomical Society, which ends today. The work follows up and lends weight to the discovery of Proxima c announced earlier this year by a team led by Mario Damasso of Italy’s National Institute for Astrophysics (INAF), which had used radial velocity methods to observe the star. We need further work, however, to say that Proxima c has been confirmed, as Dr. Benedict explained in an email this morning.
But first, let’s straighten out a question of identity. Yesterday, when discussing the confirmation of habitable zone world Proxima b, we talked about a second signal in data culled by the ESPRESSO spectrograph. If the second ESPRESSO signal does turn out to be a planet, it will be a third Proxima Centauri planet, not Proxima c. That signal does not rise to candidate planet status, nor does the ESPRESSO team claim it as such, but it suggests a minimum mass about a third of Earth’s at an orbital distance inside Proxima b in a five-day orbit.
Proxima c as studied by Benedict is a different world entirely. What the new work addresses is the Damasso finding of a planet in a 1,907-day orbit at a distance of 1.5 AU, well outside the star’s habitable zone. Seeing Damasso’s work, Benedict made the decision to re-examine data he had collected on Proxima Centauri using the Fine Guidance Sensors (FGS) on the Hubble Space Telescope. This is a classic case of tapping old data, as the Hubble work was done in the 1990s.
Image: Fritz Benedict, emeritus senior research scientist with the University of Texas at Austin’s McDonald Observatory. Credit: McDonald Observatory
And while Damasso used radial velocity methods (examining the star’s movements toward and away from Earth as influenced by planetary companions), the Hubble FGS, which were designed for pointing accuracy, allowed Benedict to use astrometry, the measurement of the positions and motions of stars. In the earlier study, Benedict worked with Barbara MacArthur, also at McDonald Observatory, to look for planets with orbital periods of 1,000 days or fewer, and found none. A re-investigation of the dataset looking for planets in longer orbital periods turned up the signal at 1,907 days.
Benedict then turned to images collected by INAF’s Raffaele Gratton using the SPHERE instrument on the Very Large Telescope in Chile, which showed what could be Proxima c at several points in its orbit. In An Image of Proxima c?, I ran a figure from the Gratton paper reproduced below, along with the paper’s caption.
Image: This is Figure 2 from the paper. The SPHERE images were acquired during four years through a survey called SHINE, and as the authors note, “We did not obtain a clear detection.” The figure caption in the paper reads like this: Fig. 2. Individual S/N maps for the five 2018 epochs. From left to right: Top row: MJD 58222, 58227, 58244; bottom row: 58257, 58288. The candidate counterpart of Proxima c is circled. Note the presence of some bright background sources not subtracted from the individual images. However, they move rapidly due to the large proper motion of Proxima, so that they are not as clear in the median image of Figure 1. The colour bar is the S/N. S/N detection is at S/N=2.2 (MJD 58222), 3.4 (MJD 58227), 5.9 (MJD 58244), 1.2 (MJD=58257), and 4.1 (MJD58288). Credit: Gratton et al.
What we now have on Proxima c, then, is the result of Hubble astrometry, radial velocity studies (Damasso et al.) and direct imaging (Gratton et al.), all of which allowed Benedict to refine the mass of the planet to about 7 times that of Earth. Older data serve us well.
“Basically, this is a story of how old data can be very useful when you get new information,” Benedict said. “It’s also a story of how hard it is to retire if you’re an astronomer, because this is fun stuff to do!”
Amen to that. Indeed, it’s hard to see how any astronomers specializing in planets around other stars could bring themselves to retire as we go ever deeper into what will surely be described as the ‘golden age’ of exoplanet studies.
When I contacted Dr. Benedict this morning, he told me that for now, his official statement on Proxima Centauri c is “A Preliminary Mass for Proxima Centauri C,” in Research Notes of the AAS Volume 4, Issue 4, id.46 (full text).
Because the individual detections from FGS, radial velocity and imaging, are all at the limit of detection, we should look toward more observations from SPHERE and future Gaia data on orbital perturbation at Proxima Centauri to serve as a further check for confirmation.
