Centauri Dreams

Objects of interstellar origin in our own Solar System continue to draw attention. Comets from other stars like 2I/Borisov give us the chance to delve into the composition of different stellar systems, while the odd ‘Oumuamua still puzzles astronomers. Comet? Asteroid?

Now we have a paper from Fathi Namouni (Observatoire de la Côte d’Azur, France) and Maria Helena Morais (Universidade Estadual Paulista, Brazil) targeting what the duo believe to be a population of asteroids captured from other stars in the distant past. Published in Monthly Notices of the Royal Astronomical Society, the paper relies on a high-resolution statistical search for stable orbits, ‘unwinding’ these orbits back in time to explain the location of certain Centaurs, asteroids moving perpendicular to the orbital plane of the planets and other asteroids.

Centaurs, most of which do not occupy such extreme positions, are a population of asteroids moving between the outer planets in what have until now been considered unstable orbits. The ones Namouni and Morais focus on are not recent interstellar ‘interlopers’ like ‘Oumuamua and Borisov, but objects that may have been drawn into the Sun’s gravitational pull at a time when the Solar System was still in formation, some 4.5 billion years ago. In those days, the Sun would have been part of a star cluster, with many young stars in close proximity. The authors believe they can identify 19 asteroids that once orbited other stars and now orbit the Sun.

Image: A stellar nursery in the Lobster Nebula (NGC6357), where star systems exchange asteroids as our Solar System is thought to have done 4.5 billion years ago. Credit: ESO / VVV Survey / D. Minniti. Acknowledgement: Ignacio Toledo (CC BY 4.0).

The authors delve into the dynamical evolution of Centaurs through statistical searches for stable orbits, to find out whether any could have survived from the days of the Solar System’s formation. In 2018, Namouni and Morais produced a paper using these methods that led to the identification of a retrograde co-orbital asteroid of Jupiter called (514107) Ka‘epaoka‘awela as an object of likely interstellar origin. The new study extends their simulations to the past orbits of 17 high-inclination Centaurs and two trans-Neptunian objects (2008 KV42 and (471325) 2011 KT19) with polar orbits. From the paper:

The statistical distributions show that their orbits were nearly polar 4.5?Gyr in the past, and were located in the scattered disc and inner Oort cloud regions. Early polar inclinations cannot be accounted for by current Solar System formation theory as the early planetesimal system must have been nearly flat in order to explain the low-inclination asteroid and Kuiper belts. Furthermore, the early scattered disc and inner Oort cloud regions are believed to have been devoid of Solar system material as the planetesimal disc could not have extended far beyond Neptune’s current orbit in order to halt the planet’s outward migration. The nearly polar orbits of high-inclination Centaurs 4.5?Gyr in the past therefore indicate their probable early capture from the interstellar medium.

The conclusion that these high-inclination Centaurs had polar inclinations at the time of the Solar System’s formation can be tested by further observations to firm up their orbits in comparison to the simulated results. It’s a natural leap from that possibility to the Sun’s birth cluster of stars, which would have supplied plentiful source material in the form of asteroids and comets available for capture. Thus the idea that all Centaurs are on unstable orbits is contradicted by 4.5 billion year orbits for high-inclination Centaurs, as well as some lower-inclination Centaurs like Chiron, which the authors also factored into their computations:

Either Chiron is an outlier that belonged to the planetesimal disc and whose cometary activity by some unknown mechanism increased its inclination far above the planetesimal disc’s mid-plane, or it could be itself of interstellar origin. Asteroid capture in the Sun’s birth cluster does not necessarily favour objects whose orbits have or evolve to polar or high-inclination retrograde motion (Hands et al. 2019). An astronomical illustration of the principle may be found in the distribution of the irregular satellites of the giant planets. Applying the high-resolution statistical stable orbit search to low-inclination Centaurs is likely to shed light on the possible common capture events that occurred in the early Solar system.

The paper is Namouni et al., “An Interstellar Origin for High-Inclination Centaurs,” Monthly Notices of the Royal Astronomical Society Volume 494, Issue 2 (May 2020), pp. 2191–2199. Abstract / preprint. And on the interesting question of Jupiter’s retrograde co-orbital asteroid and its possible interstellar origins, the paper is Namouni and Morais, “An interstellar origin for Jupiter’s retrograde co-orbital asteroid,” Monthly Notices of the Royal Astronomical Society: Letters 21 May 2018 (abstract).

I’m keeping an eye on the recent attention being paid to Proxima Centauri c, the putative planet whose image may have been spotted by careful analysis of data from the SPHERE (Spectro-Polarimetric High-Contrast Exoplanet Research) imager mounted on the European Southern Observatory’s Very Large Telescope. A detection by direct imaging of a planet found first by radial velocity methods would be a unique event, and the fact that this might be a planet in the nearest star system to our own makes the story even more interesting.

I hasten to add that this is not Proxima b, the intriguing planet in the star’s habitable zone, but the much larger candidate world, likely a mini-Neptune, that has been identified but not yet confirmed. Proxima Centauri c could use a follow-up to establish its identity, and this direct imaging work would fit the bill if it holds up. But for now, the planet is still a candidate rather than a known world. From the paper:

While we are not able to provide a firm detection of Proxima c, we found a possible candidate that has a rather low probability of being a false alarm. If our direct NIR/optical detection of Proxima c is confirmed (and the comparison with early Gaia results indicates that we should take it with extreme caution), it would be the first optical counterpart of a planet discovered from radial velocities. A dedicated survey to look for RV planets with SPHERE lead to non-detections (Zurlo et al. 2018b).

But we’re not far enough along to spend much time on this in these pages — a great deal of follow-up work will be needed to nail down what is at best an unlikely catch. I call it that because it strains credulity to believe that we would find a planet whose mass suggests a world far less bright than this one is (if indeed it is a planet), with a luminosity that demands something like a huge ring system to explain it. Other explanations will need to be ruled out, and that’s going to take time. The radial velocity work on Proxima Centauri c points to a world of a minimum six Earth masses, orbiting 1.5 AU out. But if this direct imaging work has indeed identified (and confirmed) Proxima c, it is a planet with a most unusual makeup:

If real, the detected object (contrast of about 16-17 mag in the H-band) is clearly too bright to be the RV [radial velocity] planet seen due to its intrinsic emission; it should then be circumplanetary material shining through reflected star-light. In this case we envision either a conspicuous ring system (Arnold & Schneider 2004), or dust production by collisions within a swarm of satellites (Kennedy & Wyatt 2011; Tamayo 2014), or evaporation of dust boosting the planet luminosity (see e.g. Wang & Dai 2019). This would be unusual for extrasolar planets, with Fomalhaut b (Kalas et al. 2008), for which there is no dynamical mass determination, as the only other possible example. Proxima c candidate is then ideal for follow-up with RVs observations, near IR imaging, polarimetry, and millimetric observations.

So good for Raffaele Gratton (INAF – Osservatorio Astronomico di Padova, Italy) and colleagues for pursuing this investigation and for suggesting the numerous ways it can be approached with various follow-up methods. And kudos to Mario Damasso (Astrophysical Observatory of Turin) as well. Damasso was lead author of the discovery paper on Proxima Centauri c, and it was he who suggested to Gratton that the SPHERE instrument might just be able to detect it.

Now we have a wait on our hands before we have anything definitive. At this point we have a possible detection that is tantalizing but definitely no more than tentative. Meanwhile, here’s a figure from the paper that gives an idea what Gratton and team are talking about.

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.

The paper is Gratton et al., “Searching for the near infrared counterpart of Proxima c using multi-epoch high contrast SPHERE data at VLT,” accepted at Astronomy & Astrophysics (abstract).

What an exceptional system the one around HD 158259 is! Here we have six planets, uncovered with the SOPHIE spectrograph at the Haute-Provence Observatory in the south of France, with the innermost world also confirmed through space-based TESS observations. Multiple things jump out about this system. For one thing, all six planets are close to, but not quite in, a 3:2 resonance. That ‘close to’ tells the tale, for researchers believe there are clues to the formation history of the system within their observations of this resonance.

Image: In the planetary system HD 158259, all pairs of subsequent planets are close to the 3:2 resonance : the inner one completes about three orbits as the outer completes two. Credit & Copyright: UNIGE/NASA.

The primary, HD 158259, is itself interesting, in that it’s a G-class star about 88 light years out, an object just a little more massive than our Sun. But tucked well within the distance of Mercury from the Sun we find all six of the thus far discovered planets. In fact, the outermost planet orbits at a distance 2.6 times smaller than Mercury’s, making this a compact arrangement indeed. Five of the planets are considered ‘mini-Neptunes’, while the sixth is a ‘super-Earth.’

The innermost world masses about twice the mass of Earth, while the five outer planets weigh in at about six times Earth’s mass each. The 3:2 resonance detected here runs through the entire set of planets, so that as the planet closest to the star completes three orbits, the next one out completes two, or close to it (remember, this is an ‘almost resonant’ situation). And so on — the second planet completes three orbits while the third completes about two.

Nathan Hara (University of Geneva), who led the study, likens the resonance to music, saying “This is comparable to several musicians beating distinct rhythms, yet who beat at the same time at the beginning of each bar.” The researchers involved (who used, by the way, the same telescope deployed by Michel Mayor and Didier Queloz in their ground-breaking detection of 51 Pegasi b in 1995, though with added help from SOPHIE) believe that the ‘almost resonances’ here suggest that what had been a tight resonance was disrupted by synchronous migration.

In other words, the six planets would have formed further out from the star and then moved inward together. As Hara puts it:

“Here, ‘about’ is important. Besides the ubiquity of the 3:2 period ratio, this constitutes the originality of the system. Furthermore, the current departure of the period ratios from 3:2 contains a wealth of information. With these values on the one hand, and tidal effect models on the other hand, we could constrain the internal structure of the planets in a future study. In summary, the current state of the system gives us a window on its formation.”

And here’s how the paper deals with the issue;

…period ratios so close to 3:2 are very unlikely to stem from pure randomness. It is therefore probable that the planets underwent migration in the protoplanetary disk, during which each consecutive pair of planets was locked in 3:2 MMR [mean-motion orbital resonance]. The observed departure of the ratio of periods of two subsequent planets from exact commensurability might be explained by tidal dissipation, as was already proposed for similar Kepler systems (e.g., Delisle & Laskar 2014). Stellar and planet mass changes have also been suggested as a possible cause of resonance breaking (Matsumoto & Ogihara 2020). The reasons behind the absence of three-body resonances, which are seen in other resembling systems (e.g., Kepler-80, MacDonald et al. 2016), are to be explored.

It’s interesting that while other systems with compact planets in near-resonance conditions have been detected (TRAPPIST-1 is the outstanding example, I suppose, but as we see above, Kepler-80 also fits the bill), this is the first to have been found through radial velocity methods. According to the authors, the method demands a high number of data points and accurate accounting of possible instrumental or stellar noise in the signal. In this work, 290 radial velocity measurements were taken, and supplemented by the TESS transit data on the inner planet.

Compact systems with multiple planets on close orbits do not appear only among M-dwarfs like HD 158259. Kepler-223, for example, is a G-class star with four known planets. Here the orbital periods are 7, 10, 15 and 20 days respectively. The Dispersed Matter Planet Project (DMPP) has turned up data on an F-class star (HD 38677) with four massive planets with orbital periods ranging from 2.9 to 19 days. The ancient Kepler-444 is a K-class star with five evidently rocky worlds orbiting the star in less than ten days.

Rather than the size of the star, at least one recent paper argues that metallicity is a key factor in producing compact systems (Brewer et al., (2018) “Compact multi-planet systems are more common around metal-poor hosts,” Astrophys J 867:L3). Clearly we have much to learn about planet formation and migration in compact systems. Such systems are near or below the current detection limits of radial velocity surveys — this is where the work on HD 158259 truly stands out — but they are good targets for transit studies. It will be instructive to see what TESS comes up with as it continues its work.

The paper is Hara et al., “The SOPHIE search for northern extrasolar planets. XVI. HD 158259: A compact planetary system in a near-3:2 mean motion resonance chain,” Astronomy & Astrophysics Vol. 636, L6 (April 2020). Abstract / preprint.

We’re learning interesting things about 2I/Borisov, the first interstellar comet discovered entering our Solar System (‘Oumuamua may have been a comet as well, but the lack of an active gas and dust coma makes it hard to say for sure). Moving at 33 kilometers per second, 2I/Borisov is on a trajectory clearly indicating an interstellar origin. Now two different studies have shown that in terms of composition, the visiting object is unlike most of the comets found in our own system.

Both the Hubble Space Telescope and the Atacama Large Millimeter/submillimeter Array (ALMA) have found levels of carbon monoxide (CO) higher than expected, a concentration greater than any comet yet detected within 2 AU of the Sun (about 300 million kilometers). The ALMA team finds the CO concentration to be somewhere between 9 and 26 times higher than inner system comets, while Hubble sees levels at least 50 percent more abundant than the average of comets in the inner system. Dennis Bodewits (Auburn University) is lead author of the paper on the Hubble work, which appears in Nature Astronomy:

“The amount of carbon monoxide did not drop as expected as the comet receded from the Sun. This means that we are seeing the primitive layers of the comet, which really reflect what this object is made of. Because of the abundance of carbon monoxide ice that survived so close to the Sun, we think that comet Borisov comes from a much colder place and from a very different debris disk around a star than our own.”

Image: Comet 2I/Borisov, captured here in two separate images from NASA’s Hubble Space Telescope, including one with a background galaxy (left), is the second interstellar object known to enter our Solar System. New analysis of Borisov’s bright, gas-rich coma indicates the comet is much richer in carbon monoxide gas than water vapor, a characteristic very unlike comets from our Solar System. Credit: NASA, ESA and D. Jewitt (UCLA).

2I/Borisov isn’t completely off the charts — there is one Oort Cloud comet, the recently discovered C/2016 R2 (PanStarrs), that showed even higher CO levels than Borisov when it reached a distance of 2.8 AU from the Sun. But the Borisov results are intriguing and give us a preliminary benchmark against which to measure interstellar objects discovered in the future. The Hubble work was performed using the instrument’s Cosmic Origins Spectrograph, sensitive to ultraviolet, with data taken in four periods from Dec. 2019 to Jan. 2020. NASA’s Swift satellite was able to confirm Hubble’s readings with measurements taken over the same period.

Carbon monoxide is volatile enough that it begins to sublimate (turn from ice to gas) far out in the Solar System, perhaps as much as three times the distance between Pluto and the Sun. Water, on the other hand, remains an ice until somewhere between Mars and the inner edge of the main asteroid belt, and water is the prevalent signature of comets in the inner system. Adds Bodewits: “Borisov’s large wealth of carbon monoxide implies that it came from a planet formation region that has very different chemical properties than the disk from which our solar system formed.”

In a second study reported in the same issue of Nature Astronomy, scientists using ALMA data collected in the same time frame as Hubble’s (15-16 December 2019) detected both hydrogen cyanide (HCN) and CO in Borisov. The HCN showed up at levels that were not surprising to researchers, but the CO amounts had researchers speculating on the object’s origins. Martin Cordiner, along with Stefanie Milam, led an international team at NASA GSFC:

“Most of the protoplanetary disks observed with ALMA are around younger versions of low-mass stars like the Sun. Many of these disks extend well beyond the region where our own comets are believed to have formed, and contain large amounts of extremely cold gas and dust. It is possible that 2I/Borisov came from one of these larger disks… This is the first time we’ve ever looked inside a comet from outside our solar system, and it is dramatically different from most other comets we’ve seen before.”

Image: ALMA observed hydrogen cyanide gas (HCN, left) and carbon monoxide gas (CO, right) coming out of interstellar comet 2I/Borisov. The ALMA images show that the comet contains an unusually large amount of CO gas. ALMA is the first telescope to measure the gases originating directly from the nucleus of an object that travelled to us from another planetary system. Credit: ALMA (ESO/NAOJ/NRAO), M. Cordiner & S. Milam; NRAO/AUI/NSF, S. Dagnello.

The Hubble team considers a carbon-rich circumstellar disk around a cool red dwarf as one possibility for 2I/Borisov’s birthplace. Here, gravitational interactions with a planet orbiting in the outer reaches of the system could have ejected the comet. “These stars have exactly the low temperatures and luminosities where a comet could form with the type of composition found in comet Borisov,” says John Noonan (Lunar and Planetary Laboratory, University of Arizona, Tucson).

This is interesting speculation, but the real news here is that this is the first measurement of the carbon monoxide composition of a comet from another star. The work points to the entirely new area of exoplanetology that is opening up, the study of debris from other stellar systems that will be further enabled as new telescopes and survey methods come online. Future objects should help us clarify how to use such data as we explore planet formation around other stars.

The papers are Bodewits et al., “The carbon monoxide-rich interstellar comet 2I/Borisov,” Nature Astronomy 20 April 2020 (abstract) and Cordiner et al., “Unusually high CO abundance of the first active interstellar comet,” in the same issue of Nature Astronomy (full text).

What intrigues me about Kepler-1649c, a newly discovered planet thrust suddenly into the news, isn’t the fact that it’s potentially in its star’s habitable zone, nor that it is close to being Earth-sized (1.06 times Earth’s radius). Instead, I’m interested in the way it was found. For this is a world turned up in exhaustive analysis of data from the original Kepler mission. Bear in mind that data from the original Kepler field ceased being gathered a full seven years ago.

I share Jeff Coughlin’s enthusiasm on the matter. Coughlin is an astronomer affiliated with the SETI Institute who is a co-author on the new paper, which appears in Astrophysical Journal Letters. One of the goals of the mission that began as Kepler and continued (on different star fields) as K2 was to find the fraction of stars in the galaxy that have planets in the habitable zone, using transit methods for initial detection and radial velocity follow-up on Earth. Of the new find, made with an international team of scientists, Coughlin says:

“It’s incredible to me that we just found [Kepler-1649c] now, seven years after data collection stopped on the original Kepler field. I can’t wait to see what else might be found in the rich dataset from Kepler over the next seven years, or even seventy.”

Yes, because we’re hardly through with a dataset that has taken in close to 200,000 stars. What the new planet reminds us is that the initial detection stage of Kepler’s work long ago ceded to analysis of how many of the acquired signals are due to systematic effects in the instrumentation, or variable stars, or perhaps binaries, for so many things can appear to be the signature of a planet when they are not. Making computer algorithms to automate this work has occurred only after intensive human study to learn the best ways to distinguish between signals.

But consider: The algorithm, called Robovetter, that was used to distinguish false positives was working on data in which only 12 percent of the transit signals turned out to be planets. We have false positives galore, out of which we now get the Kepler False Positive Working Group to serve as a double-check on the algorithm’s work. The KFPWG is a dedicated team indeed, taking years to review the thousands of signals from the original Kepler dataset. Kepler-1649c comes out of this review, a world that had been misclassified by the automated methods earlier on. Clearly, humans and machines learn from each other as they make such difficult calls.

Image: A comparison of Earth and Kepler-1649c, an exoplanet only 1.06 times Earth’s radius. Credit: NASA/Ames Research Center/Daniel Rutter.

I yield to the experts when it comes to habitability, and trust our friend Andrew LePage will weigh in soon with his own analysis. What we do know is that this planet receives about 75 percent of the amount of light Earth receives from the Sun, though in this case the star in question is a red dwarf of spectral type M5V, the same as Proxima Centauri, fully 300 light years out. As always with red dwarfs, we bear in mind that stellar flares are not uncommon. The planet orbits its star every 19.5 days. We have no information about its atmosphere.

Even so, lead author Andrew Vanderburg (University of Texas at Austin) notes the potential for habitability:

“The more data we get, the more signs we see pointing to the notion that potentially habitable and Earth-sized exoplanets are common around these kinds of stars. With red dwarf stars almost everywhere around our galaxy, and these small, potentially habitable and rocky planets around them too, the chance one of them isn’t too different than our Earth looks a bit brighter.”

There is a planet on an orbit interior to Kepler-1649c, a super-Earth standing in relation to the former much as Venus does to the Earth. The two demonstrate an uncommon nine-to-four orbital resonance, one that hints at the possibility of a third planet between them. This would give us a pair of three-to-two resonances, a much more common scenario, though no trace of the putative world shows up in the data. If there, it is likely not a transiting planet.

I rather like the image that comes out of NASA Ames, where the Kepler effort is managed, but I like it as a kind of science fictional art, the sort of thing to be found on SF paperbacks. It’s evocative and suggestive, but of course we have no idea what this world really looks like.

Image: An illustration of what Kepler-1649c could look like from its surface. Credit: NASA/Ames Research Center/Daniel Rutter.

The paper is Vanderburg et al., “A Habitable-zone Earth-sized Planet Rescued from False Positive Status,” Astrophysical Journal Letters Vol. 893, No. 1 (1 April 2020). Abstract.