Centauri Dreams

The SAINT-EX telescope, operated by NCCR PlanetS, produces a nice resonance as I write this morning. The latter acronym stands for the National Centre of Competence in Research PlanetS, operated jointly by the University of Bern and the University of Geneva. The former, SAINT-EX, identifies a project called Search And characterIsatioN of Transiting EXoplanets, and the team involved explicitly states that they shaped their acronym to invoke Antoine de Saint-Exupéry, legendary aviator and author of, among others, Wind, Sand and Stars (1939), Night Flight (1931) and Flight to Arras (1942).

I’ve talked about Saint-Exupéry now and again throughout the history of Centauri Dreams, not only because he was an inspiration for my own foray into flying, but also because for our interstellar purposes he is credited with this inspirational thought:

“If you want to build a ship, don’t drum up the men to gather wood, divide the work and give orders. Instead, teach them to yearn for the vast and endless sea.”

I say Saint-Exupéry is ‘credited’ with this because it seems to be a condensation of a longer passage that appeared in his 1948 title La Citadelle and I’ve never been able to track the oft-quoted short version down to an original in this specific form. No matter, it’s a grand thought and it plays to our passions as explorers and mappers of new worlds.

So good for NCCR PlanetS for the nod to a favorite writer, and thanks for its recent work at TOI-1266, a red dwarf now known to host at least two planets. The confirmation was achieved with the SAINT-EX robotic 1-meter telescope that the project operates in Mexico; the paper was recently published in Astronomy & Astrophysics. TOI identifies a TESS Object of Interest, meaning the object had been considered promising for follow-up work to validate the candidate signatures as planets. The paper on TOI-1266 provides the needed confirmation.

Here’s what we know: TOI-1266 b is an apparent sub-Neptune about two and a half times the diameter of the Earth that orbits the primary in 11 days. TOI-1266 c is closer in size to a ‘super-Earth’ in being about one and a half times the size of our planet in a 19-day orbit.

The orbits are circular, co-planar and stable. Referring to the more than 3000 transiting planets thus far identified, which includes 499 with more than a single transiting world, the authors discuss distinctive planetary populations. Both planets at TOI-1266 are found at what lead author Brice-Olivier Demory calls the ‘radius valley,’ for reasons explained in the paper:

This large sample of transiting exoplanets allows for in-depth exploration of the distinct exoplanet populations. One such study by Fulton et al. (2017) identified a bi-modal distribution for the sizes of super-Earth and sub-Neptune Kepler exoplanets, with a peak at ?1.3 R_? and another at ?2.4 R_?. The interval between the two peaks is called the radius valley and it is typically attributed to the stellar irradiation received by the planets, with more irradiated planets being smaller due to the loss of their gaseous envelopes.

Image: Size of TOI-1266 system compared to the inner solar system at a scale of one astronomical unit, the distance between the Earth and the Sun. The orbital distances of the two exoplanets discovered around the star TOI-1266, which is half of the size of the Sun, are smaller than Mercury’s orbital distance. TOI-1266 b, the closest planet to the star at a distance of 0.07 astronomical units, has a diameter of 2.37 times that of Earth’s and is therefore considered a sub-Neptune. TOI-1266 c, at 0.01 astronomical units from its star and with 1.56 times the Earth’s diameter, is considered a super-Earth. For each planetary system, the star’s diameter and the orbital distances to its planets are shown in scale. The relative diameter of all planets of both systems are on scale, being TOI-1266 b the largest planet and Mercury the smallest. The zoom-in to TOI-1266, in the lower part of the image, shows that the irradiation received by TOI-1266 c from its star is 21% larger than the irradiation received by Venus from the Sun in the upper part of the image. © Institute of Astronomy, UNAM/ Juan Carlos Yustis.

It’s useful to have a super-Earth and a sub-Neptune in the same system because this allows for tighter constraints on formation models. The paper argues that this will be “a key system to better understand the nature of the radius valley around early to mid-M dwarfs.”

It’s also interesting that TOI-1266’s small size and relative proximity (about 117 light years) make it a possible target for atmospheric studies with future instruments. The authors show that the James Webb Space Telescope should be able to operate well above the observatory’s noise floor level in performing transmission spectroscopy here (i.e., viewing changes in the star’s light as the either planet moves first in front of and then is eclipsed by the star). The TOI-1266 planets compare favorably to TRAPPIST-1b on this score, and the host star shows no evidence of significant starspots that might distort the signal of the exoplanet atmospheres.

Image: Direct comparison among the planets of TOI-1266 system with the interior planets of the solar system. © Institute of Astronomy, UNAM / Juan Carlos Yustis.

The paper is Brice-Olivier Demory et al., “A super-Earth and a sub-Neptune orbiting the bright, quiet M3 dwarf TOI-1266,” Astronomy & Astrophysics Volume 642 (2 October 2020). Abstract.

Gas giant planets in orbits similar to Jupiter’s are a tough catch for exoplanet hunters. They’re far enough from the star (5 AU in the case of Jupiter) that radial velocity methods are far less sensitive than they would be for star-hugging ‘hot Jupiters.’ A transit search can spot a Jupiter analogue, but the multi-year wait for the proper alignment is obviously problematic.

Still, we’d like to know more, because the gravitational influence of Jupiter may have a crucial role in deflecting asteroids and comets from the inner system, thus protecting terrestrial worlds like ours. If this is the case, then we need to ask whether we just lucked out by having Jupiter where it is, or whether there is some mechanism that makes the presence of a gas giant in a kind of ‘protective’ outer orbit likely when rocky worlds inhabit the inner system.

Enter the computer simulations run by Martin Schlecker at the Max Planck Institute for Astronomy (MPIA) in Heidelberg. Working with scientists at the University of Bern and the University of Arizona, Schlecker has run simulations of 1000 planetary systems evolving around Sun-like stars. In this case, ‘Sun-like’ means solar-type stars with the same mass as our own Sun. The simulated planetary systems are derived through a model known as the Generation III Bern global model of planetary formation and evolution, and the team has used the results to make a statistical comparison with a sample of observed exoplanet systems.

Schlecker calls gas giants in more distant orbits ‘cold Jupiters.’ They’re beyond the snowline, so that water can exist in the form of ice. The paper homes in on ‘super-Earths’ more massive than our own planet but far less massive than gas giants, which are estimated to orbit as many as 50% of stars in class F, G and K. In question is how cold Jupiters affect the formation of these inner super-Earths, and if in fact they make it easier for super-Earths to survive. Some recent studies have found a positive correlation between super-Earths and cold Jupiters. Is it sound?

Image: Artistic impression of a planetary system with two super-Earths and one Jupiter in orbit around a Sun-like star. Simulations show that massive protoplanetary disks in addition to rocky super-Earths with small amounts of ice and gas often form a cold Jupiter in the outer regions of the planetary systems. Credit: MPIA graphics department.

The team’s goal is a “population synthesis of multi-planet systems from initial conditions representative of protoplanetary disks in star forming regions.” The computer model tracks how systems starting from random initial conditions evolve over several billion years as planets coalesce and interact with other objects in the system, with the possibility of collisions or ejections as orbits change due to the gravitational interactions factored in. You can see that with the difficulty in observing actual cold Jupiters in such settings, computer simulations are the way to proceed as the tools of observation are tuned up for future detections.

A hot Jupiter would be likely to disrupt planetary orbits, perhaps preventing the formation of some inner worlds altogether, but a cold Jupiter is far enough from the star that we can expect systems with inner super-Earths to exist, and indeed, some studies suggest that systems with a cold Jupiter often contain a super-Earth. But the simulations of Schlecker’s team indicate that only about a third of cold Jupiters occur in systems accompanied by at least one super-Earth. The correlation of inner super-Earths and cold Jupiters is there, but somewhat weaker than has been found through actual observation.

A possible explanation for the discrepancy between observation and simulation: Gas giant migration involving ‘warm Jupiters’ on intermediate orbits, causing super-Earth collisions or ejections. Here we can look to the parameters of the simulations. If the tendency of the simulated gas giants to migrate is slightly lowered, more inner super-Earths remain in numbers that are compatible with what we have recorded with our telescopes and spectrographs.

Here the limited number of actually observed systems with gas giant and super-Earths becomes an issue. Whereas the study’s simulations find a tendency to produce systems with both a cold Jupiter and at least one dry super-Earth (“with little water or ice, and a thin atmosphere at most,” as Schlecker notes), we only have a small number of systems (24) out of the 3200 planetary systems currently known that follow this configuration. Schlecker also notes that the Earth is comparatively dry — “…the Earth is, despite the enormous oceans and the polar regions, with a volume fraction for water of only 0.12% altogether a dry planet.” What we don’t find observationally are ice-rich super-Earths with a cold Jupiter in the same system.

Image: Schematic diagram of the scenarios of how according to the analyzed simulations icy super-Earths (a) or rocky (ice-poor) super-Earths form together with a cold Jupiter (b). The mass of the protoplanetary disk determines the result. Credit: Schlencker et al./ MPIA.

The mass of the protoplanetary disk may be the key. A massive enough disk can form inner rocky planets with cold gas giants beyond the snowline, with the rocky planets being poor in ice and gas. Outer super-Earths that form ice-rich would be unable to migrate inward because of the giant planet. In disks of medium mass, by contrast, material to produce warm super-Earths is limited, and gas giants beyond the snowline give way to super-Earths abundant in ice.

In the authors’ words:

We find a difference in the bulk composition of inner super-Earths with and without cold Jupiters. High-density super-Earths point to the existence of outer giant planets in the same system. Conversely, a present cold Jupiter gives rise to rocky, volatile-depleted inner super-Earths. Birth environments that produce such dry planet cores in the inner system are also favorable for the formation of outer giants, which obstruct inward migration of icy planets that form on distant orbits. This predicted correlation can be tested observationally.

and…

…low-mass solid disks tend to produce only super-Earths but no giant planets. Intermediate-mass disks may produce both super-Earths and cold Jupiters. High-mass disks lead to the destruction of super-Earths and only giants remain.

The mass of the protoplanetary disk is thus crucial, with super-Earth occurrence in combination with cold Jupiters rising with increasing disk mass, then dropping for disks massive enough to become dominated by giant planets. Interestingly, the study indicates that host stars of high-metallicity tend to have planetary systems with warm giant planets that can wreak havoc on inner planets in the system.

We’re in that region of the discovery space where we’re building theories that can be tested against future observations. Thus the next generation of space telescopes like the James Webb Space Telescope, as well as the Extremely Large Telescope from the European Southern Observatory. Do the simulations of Schlecker and team hold up? Do they show a disposition for planets of Earth size, rather than the larger super-Earths, to exist in systems with cold Jupiters?

We won’t know, the scientist acknowledges, until we have the ability to detect planets like ours in large numbers. For now, the only numbers we can begin to run are those of the larger super-Earth population, and even here, we’re still building the statistical sample. What the new instrumentation shows in coming years will help us fine-tune these simulated results.

The paper is Schlecker et al., “The New Generation Planetary Population Synthesis (NGPPS). III. Warm super-Earths and cold Jupiters: A weak occurrence correlation, but with a strong architecture-composition link,” in process at Astronomy & Astrophysics (abstract).

I’m startled by the findings in a new paper from Dominique Segura-Cox (Max Planck Institute for Extraterrestrial Physics), who argues that based on the evidence of one infant system, we may have planet formation all wrong, at least in terms of when it occurs. The natural assumption is that the star appears first, the planets then accruing mass from within the circumstellar disk. But Segura-Cox and team have found a system in which planet and star seem to be forming all but simultaneously. IRS 63 is a protostar about 470 light years out that is less than half a million years old. Swathed in gas and dust, the star is still gathering mass, but evidence from the disk suggests that the planets have already begun to form.

One reason for the surprise factor here is that we’ve looked at many young stellar systems and their disks, most of them at least one million years old, and the assumption has been that the stars were well along in their own formation process before the planets began to build. Ian Stephens (Center for Astrophysics | Harvard & Smithsonian), a co-author on this paper, is quite clear on the matter: “Traditionally it was thought that a star does most of its formation before the planets form, but our observations showed that they form simultaneously.”

Image: The dense L1709 region of the Ophiuchus Molecular Cloud, mapped by the Herschel Space Telescope, which surrounds and feeds material to the much smaller IRS 63 proto-star and planet-forming disk (location marked by the black cross). Credit: MPE/D. Segura-Cox; Herschel data from ESA/Herschel/SPIRE/PACS/D. Arzoumanian.

Planets and host star as siblings? It takes a bit of getting used to, but the Segura-Cox team is finding gaps and rings, the rings accumulating dust suitable for planet formation, within the IRS 63 disk. The paper summarizes earlier disk observations at other stars, where the gaps and rings found in the disks have generally appeared in so-called Class II disks with ages of one million years or more — ALMA (Atacama Large Millimeter/submillimeter Array) has turned up more than 35 of these — with gaps showing little or no dust emission, a fact usually explained by a planet-mass object shaping the dust into rings.

Fitting IRS 63 into this picture, we get this:

These results indicate that planet formation must be both efficient and well underway by the class II phase. Recent dust mass measurements of class II disks also indicate that observed dust depletion could be explained if substantial mass is locked into planetesimals on timescales less than 0.1-1 Myr, placing early planet formation into the younger class I phase and possibly even earlier. In comparison to the rings and gaps found in the older disks of class II objects, the annular substructures in the younger disk of IRS 63 have dust emission even in the G1 and G2 gaps and are wider and have lower contrast. In IRS 63, it is unclear whether or not sizeable planetary-mass bodies are creating these gaps.

Stephens estimates that planets start to form as early as the first 150,000 years of a system’s formation, at the earliest stages of star growth. If each of the gaps at IRS 63 is opened by a single planet, the authors estimate the first gap (G1) would require a planet of 0.47 Jupiter mass, while the second gap (G2) demands 0.31 Jupiter mass, both numbers being upper limits.

Image: The ALMA image of the young planet-forming dust rings surrounding the IRS 63 proto-star, which is less than 500,000 years old. Credit: MPE/D. Segura-Cox.

The paper notes that Jupiter is an interesting world in this kind of scenario. The disk at IRS 63 is similar in size to our own Solar System, and the mass of the proto-star not far off that of the Sun. Such early formation implies a distant new planet migrating into its present position:

For a planetary embryo made of solids to trigger runaway accretion of gaseous material—required in the core-accretion model of gas-giant planet formation—the critical mass of 10M_Earth must be met by the solid core. This condition is readily met in the young IRS 63 disk, even for a somewhat low efficiency (<10%) of core formation out of the available dust grains. The large dust mass in the outer dust disk—combined with the 27 and 51 au radii of the rings of IRS 63—is consistent with evidence that Jupiter’s core could have formed beyond 30 au in our own Solar System and later migrated inward. Giant-planet cores may often form in the exterior regions of large disks starting from the early phases of star formation.

Image: The rings and gaps in the IRS 63 dust disk compared to a sketch of orbits in our own Solar System at the same scale and orientation of the IRS 63 disk. The locations of the rings are similar to the locations of objects in our Solar System, with the inner ring about the size of the Neptune orbit and the outer ring a little larger than the Pluto orbit. Credit: MPE/D. Segura-Cox.

At IRS 63, the researchers find that about 0.5 Jupiter masses of dust are available at distances beyond 20 AU, along with unspecified amounts of gas. Planet formation at large distances from the star might involve solid material totalling at least 0.03 Jupiter masses, forming the kind of planetary core that can accrete gas and swell into a gas giant planet.

The paper is Segura-Cox, et al., “Four annular structures in a protostellar disk less than 500,000 years old,” Nature Vol. 586 (7 October 2020), pp. 228-231 (abstract).

We could use a lot more information about flare activity on M-dwarf stars, which can impact planetary atmospheres and surfaces and thus potential habitability. Thus far much has been said on the subject, but what has been lacking are details about the kinds of flares in question. It’s a serious issue given that, in order to be in the liquid water habitable zone, an M-dwarf planet has to orbit in breathtaking proximity to the host star.

Flares occur through a star’s magnetic field re-connection, which releases radiation across the electromagnetic spectrum. While flares can erode atmospheres and bathe the surface in UV flux, too few flares could actually be detrimental as well, providing as a new paper on the matter suggests, “insufficient surface radiation to power prebiotic chemistry due to the inherent faintness of M-dwarfs in the UV.”

The paper is out of the University of North Carolina, measuring a large sample of superflares in search of a clearer picture of their effect. Flares on the Sun are common enough, as witness the image below, shot in 2012, which shows the kind of coronal mass eruption (CME) associated with stellar flaring. At young M-dwarfs, we can get ‘superflares’ in energy ranges 10 to 1,000 times larger than our Sun provides, bathing a nearby planet in intense ultraviolet light.

“We found planets orbiting young stars may experience life-prohibiting levels of UV radiation, although some micro-organisms might survive,” says lead study author Ward S. Howard.

Or could flares be a driver of evolution? Just how much is too much when it comes to flaring?

Image: A beautiful prominence eruption producing a coronal mass ejection (CME) shot off the east limb (left side) of the sun on April 16, 2012. Such eruptions are often associated with solar flares, and in this case an M1 class (medium-sized) flare occurred at the same time, peaking at 1:45 PM EDT. The CME was not aimed toward Earth. The eruption was captured by NASA’s Solar Dynamics Observatory in the 304 Angstrom wavelength, which is typically colored in red. Credit: NASA/SDO/AIA.

Howard’s team used data from TESS, the Transiting Exoplanet Survey Satellite, coupled with observations from the university’s Evryscope telescope array, to make the study. Previous flare work has homed in on a relatively small number of stars in terms of flare temperatures and radiation flux. Using the largest sample of superflares ever studied in terms of temperature, the new study finds it predictive of the amount of radiation that is likely to reach a planetary surface. A statistical relationship emerges between the size of a superflare and its temperature.

Superflares are not long-lasting events, emitting most of their UV in a rapid peak that may last between 5 and 15 minutes. The TESS data, taken simultaneously with the Evryscope observations, were obtained at a two-minute cadence for 42 superflares from 27 K5-M5 dwarfs. The work extends the range of our observations, while simultaneously playing into the potential target list for the James Webb Space Telescope.

The authors note that flare emissions have usually been approximated by a 9000 K blackbody (a surface that absorbs all radiant energy falling on it). But if superflares are hotter than this, the UV emission may surge by a factor of 10 higher than would otherwise be predicted through optical observation. Only a handful of multi-wavelength observations over short periods of time have been performed, and the TESS/Evryscope work doubles the total number found in the literature, helping us understand how temperature evolves in M-dwarf superflares. 43% of the superflares studied emit above 14,000 K, 23% above 20,000 K and 5% above 30,000 K, with the hottest one observed briefly reaching an incandescent 42,000 K.

We’re dealing, in other words, with a lot of UV flux here, particularly the dangerous ultraviolet C (UV-C) wavelength. From the paper, which takes note of a 2016 superflare observed at Proxima Centauri:

If HZ planets orbiting <200 Myr stars typically receive ?120 W m^?2 and often up to 10³ W m^?2 during superflares, then significant photo-dissociation of planetary atmospheres may occur (Ribas et al. 2016; Tilley et al. 2019). As a point of comparison, the likely water loss of Proxima b is due to the long-term effects of a time-averaged XUV flux (including flares) of less than 1 W m^?2 (Ribas et al. 2016). The median value from the flares observed in YMGs [young moving group stars] is comparable to the ?100 W m^?2 of UV-C flux estimated at the distance of Proxima b during the Howard et al. (2018) Proxima superflare. While Abrevaya et al. (2020) found 10^?4 microorganisms would have survived the Proxima superflare, it is presently unclear what effects a 10× increase in UVC flux would have on the evolution and survival of life prior to 200 Myr; it is possible such high rates of UV radiation could drive pre-biotic chemistry (Ranjan et al. 2017; Rimmer et al. 2018), suppress the origin of life on worlds orbiting young M-dwarfs (Paudel et al. 2019), or not impact astrobiology at all if the timescale for life to emerge is longer than 200 Myr (Dodd et al. 2017; Paudel et al. 2019).

Addendum: Alex Tolley makes a solid point about this material in the context I gave it. Let me quote him:

Exactly so, and I appreciate the note, Alex. — PG

Three of the stars targeted in this study, the aforementioned Proxima Centauri, LTT 1445 and RR Cac AB are already known to host planets. And what of the age difference between young M-dwarfs and their presumably more sedate elder cousins? Should we preferentially target older M-dwarfs? The authors argue that it’s too early to make a call:

Although higher-mass young M-dwarfs may emit more biologically-relevant UV flux as a consequence of frequent superflares than do lower-mass young M-dwarfs, we do not confirm that more UV-C flux from early M-dwarf superflares consistently reaches the HZ. The relative habitability of early versus mid M-dwarf planets is a topic for future work. In particular, the shorter active lifetimes of early M-dwarfs may allow planetary atmospheres to recover as the star ages via degassing (Moore & Cowan 2020).

So we have a useful survey of optical temperature evolution in M-dwarf superflares, demonstrating the predictive quality of flare energy and impulse. The authors intend to continue observations of future flare activity at the same 2 minute cadence, aided by the re-observation of the Evryscope flare targets at its own 20 second cadence.

The paper is Howard et al., “EvryFlare III: Temperature Evolution and Habitability Impacts of Dozens of Superflares Observed Simultaneously by Evryscope and TESS,” in process at the Astrophysical Journal (preprint).