In recent years we have witnessed a veritable flood of extrasolar planetary discoveries from NASA’s Kepler mission which watches for regular decreases in a star’s brightness caused by planetary transits. But in addition to Kepler, there are also ground-based programs looking for more easily detected planetary transits of smaller dwarf stars. One of those programs run by the European Southern Observatory (ESO), called TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope), recently found three Earth-size extrasolar planets orbiting a nearby ultracool dwarf star now called TRAPPIST-1. This first of its kind discovery is important because it suggests that Earth-size planets may be common around these very small stars. And given the relatively large decrease in the apparent brightness caused by transits of these small stars, it will be easier to study the atmospheres of such worlds using current and future instruments compared to like-size worlds orbiting larger Sun-size stars.
The 0.6-meter TRAPPIST robotic telescope at ESO’s La Silla Observatory in Chile was used to spot the transits of three Earth-size planets orbiting a nearby ultracool dwarf star. (E. Jehin/ESO)
While this discovery is important in what it can tell us about Earth-size planets, unfortunately it has also been accompanied by a fair amount of media hype as is evident in the title of ESO’s own press release announcing this discovery, “Three Potentially Habitable Worlds Found Around Nearby Ultracool Dwarf Star”. Such a sensational claim just seems too good to be true and needs to be examined in more depth to verify it.
Background
TRAPPIST-1 (also know by the 2MASS catalog designation J23062928-0502285) is an ultracool dwarf star of spectral type M8 located 39 light years away in the constellation of Aquarius with a V-magnitude of only 18.8. It is estimated that this star has a luminosity of 0.0005 times that of the Sun and a radius of 0.115 times. With a mass estimated to be only 0.08 times that of the Sun and a surface temperature of 2700 K, TRAPPIST-1 is close to being the smallest possible size for a main sequence star – any smaller and it would be a quickly cooling (and dimming) brown dwarf incapable of fusing hydrogen in its core. Determining the ages of such small stars is difficult because they evolve so slowly over their lifetimes of trillions of years but it is estimated to be in excess of a half a billion years and is probably much more.
This chart indicates the position of TRAPPIST-1 in the constellation Aquarius. Click on image to enlarge. (ESO/IAU and Sky & Telescope)
TRAPPIST-1 is one of about 60 ultracool dwarf stars in the southern skies whose brightness is being regularly monitored by the 0.6-meter TRAPPIST telescope located at ESO in La Silla, Chile. One of the objectives of this project, led by the University of Liège in cooperation with the Geneva Observatory, is to monitor the brightness of ultracool dwarf stars with spectral types later than M5 in order to detect any transits from orbiting planets as well as monitor the stars’ high-frequency variability. Stars of this sort are ideal for ground-based planetary transit searches given their small sizes which results in larger decreases in brightness for the transit of a planet of a given size (thus increasing the detectability of any planets) and the tighter orbits of their planetary systems (which increases the odds of a planet’s orbit being aligned by chance to produce an observable transit). TRAPPIST is a prototype for a more ambitious photometric survey called SPECULOOS (Search for Planets EClipsing ULtra-cOOl Stars) which will monitor the brightness of about 500 of the brightest ultracool dwarfs visible in the southern hemisphere from ESO facility in Paranal, Chile starting later in 2016.
This artist’s depiction shows the relative sizes of the Sun and the ultracool dwarf star, TRAPPIST-1. (ESO)
According to the discovery paper by the team led by Michaël Gillon (University of Liège – Belgium), the brightness of TRAPPIST-1 was monitored at a wavelength of about 0.9 μm about once every 1.2 minutes for a total of 245 hours on 62 nights between September 17 and December 28, 2015. Their light curves contained a total of 11 unambiguous transits each of which decreased the apparent brightness of the parent star by about 1%. Follow up observations were made at visible wavelengths using the two-meter Himalayan Chandra Telescope in India and in the infrared using ESO’s 8-meter Very Large Telescope in Chile and the 3.8-meter UKIRT in Hawaii. It was found that nine of the transits can be attributed to a pair of planets, TRAPPIST-1b and c, with orbital periods of 1.51 and 2.42 days.
The interpretation of the remaining two detected transits is more difficult owing to the noncontinuous nature of the available photometric data. If one planet is responsible for these two transit events, 11 different orbital periods ranging from 4.5 days to 72.8 days could satisfy the observations with 18.2 days being the most likely solution by far. It is also possible that two different planets with completely unconstrained orbits are responsible for the observation. However, Gillon et al. do not favor this scenario because the main parameters of these two transits (i.e. duration, depth and impact parameter) are so similar strongly suggesting it is the same object responsible for both transits. More data will be required to pin down the orbit of TRAPPIST-1d. Either way, the planetary system associated with this ultracool dwarf star is similar in character to those found orbiting larger red dwarf stars (see “Architecture of M-Dwarf Planetary Systems”). The characteristics of the three planets from Gillon et al. are summarized in the table below.
Properties of Planets Orbiting TRAPPIST-1
| Planet | b | c | d |
| Period (days) | 1.511 | 2.422 | 4.6 to 73 |
| Orbit Radius (AU) | 0.0111 | 0.01522 | 0.022 to 0.15 |
| Planet Radius (RE) | 1.11 | 1.05 | 1.16 |
| Seff (Earth=1) | 4.3 | 2.3 | 0.02 to 1 |
Potential Habitability
A thorough assessment of the habitability of any extrasolar planet would require a lot of detailed data on the properties of that planet, its atmosphere, its spin state and so on. Unfortunately, at this very early stage, the only information typically available to scientists about extrasolar planets is basic orbit parameters, a rough measure of its size or mass and some important properties of its sun. Combined with theoretical extrapolations of the factors that keep the Earth habitable (not to mention why our neighbors Venus and Mars are not), the best we can hope to do at this time is to compare the known properties of extrasolar planets to our current understanding of planetary habitability to determine if an extrasolar planet is “potentially habitable”. And by “habitable”, I mean habitable in an Earth-like sense where the surface conditions allow for the existence of liquid water on the planet’s surface. While there may be other worlds that might possess environments that could support life (e.g. Mars or the tidally heated oceans on the moons Europa and Enceladus), these would not be Earth-like habitable worlds of the sort being considered here.
The first attribute that provides a clue of the potential habitability of a planet is its radius. With radii of about 1.1 times that of the Earth (or RE), these three extrasolar planets are certainly Earth-size. Unfortunately, there are no measurements of the masses of these worlds currently available which could help to constrain their bulk properties and determine if they are rocky planets like the Earth or volatile-rich mini-Neptunes with no prospect of being habitable in the conventional sense. According to Gillon et al. the reflex motion of the three planets orbiting TRAPPIST-1 would be expected to produce variations in the radial velocity of the star of 0.5 to a few meters per second, depending on the planets’ compositions. Unfortunately, the star is too dim at visible wavelengths for the current generation of precision radial velocity instruments to make these measurements with the required accuracy. But future instruments operating in the infrared may be able to do so.
Gillon et al. also estimate, based on dynamical simulations, that Earth-mass planets should produce transit timing variations (TTVs) on the order of a few tens of seconds which is potentially detectable by a dedicated monitoring campaign. NASA has stated in a press release that Kepler will observe TRAPPIST-1 as part of the K2 Campaign 12 which is scheduled to run from December 15, 2016 to March 4, 2017. Based on earlier observations of similarly dim ultracool dwarfs, it appears that Kepler has sufficient sensitivity to detect the transits of TRAPPIST-1. So it seems like it will be just a matter of time before some new data including possible mass values for these extrasolar planets become available.
While we wait for astronomers to measure the masses of these new finds, we do have statistical arguments based on other extrasolar planets of know radius and mass. An analysis of the mass-radius relationship for extrasolar planets smaller than Neptune performed by Leslie Rogers (Hubble Fellow at Caltech) strongly suggests that planets transition from being predominantly rocky planets like the Earth to predominantly volatile-rich worlds like Neptune at radii no greater than 1.6 RE (for a detailed description of this work, see “Habitable Planet Reality Check: Terrestrial Planet Size Limit”). With radii comfortably below this 1.6 RE threshold, it seems quite probable that these three planets are rocky.
The Earth-like size of the three planets found orbiting TRAPPIST-1 strongly suggest that they are rocky like the Earth, as shown in this artist depiction. (ESO/M. Kornmesser/N. Risinger)
The next issue affecting the potential habitability of these worlds is the amount of energy they receive from their sun or their effective stellar flux, Seff. According to the work by Kopparapu et al. on the limits of the habitable zone (HZ) based on detailed climate and geophysical modeling, the outer limit of the HZ is conservatively defined as corresponding to the maximum greenhouse limit of a CO2-rich atmosphere where the addition of any more CO2 would not increase a planet’s surface temperature any further. For a star like TRAPPIST-1 with a surface temperature of 2700 K, this conservative outer limit for the HZ has an Seff of 0.23 corresponding to a mean orbital radius of 0.047 AU.
The inner limit of the HZ is conservatively defined by Kopparapu et al. by the runaway greenhouse limit where a planet’s temperature would soar and lose all of its water in the process. For an Earth-size planet orbiting TRAPPIST-1, this corresponds to an Seff of 0.92 or a distance of 0.023 AU. However, Gillon et al. have calculated that the inner two planets they found are most likely to be synchronous rotators with the same side perpetually facing their sun. Detailed climate modeling over the last two decades now shows that synchronous rotation is probably not the impediment to habitability as it was once thought. In fact, it has been shown that synchronous rotation actually results in an increase of the Seff for the inner edge of the HZ. According to the recent work by Yang et al., the inner edge of the HZ for a synchronous rotator orbiting a star like TRAPPIST-1 would have an Seff of 1.47 corresponding to an orbital distance of 0.018 AU.
Looking at the Seff values of 4.3 and 2.3 for TRAPPIST-1b and c, respectively, they appear to be too high to be considered to be inside the conservatively defined HZ even for a synchronous rotator. With such high Seff values, both of these worlds have likely lost all of their water and have Venus-like surface conditions, assuming they started with a more Earth-like composition. While the future study of these worlds holds much promise in characterizing the atmospheres of Earth-size exoplanets, it would seem that contrary to the claim being made in the ESO press release and some media headlines, these two worlds are not potentially habitable given what we currently know about them and about Earth-like planetary habitability. It seems much more probable that these two worlds with their Venus-like sizes, Venus-like rotation states and Seff values in excess of that of Venus are non-habitable Venus-like exoplanets instead.
This artist’s impression showing an imagined view from TRAPPIST-1d. Since it most likely orbits beyond the outer edge of the habitable zone, it is likely to be cold and desolate as depicted here. (ESO/M. Kornmesser)
The situation with TRAPPIST-1d is more ambiguous given the currently unconstrained nature of its orbit. The range of 11 possible orbits runs the gamut from being comfortably inside the HZ to well beyond the outer edge. Of the eight most likely orbital solutions found by Gillon et al. which run from 0.033 to 0.093 AU, half appear to be beyond the outer edge of the HZ including the most likely solution corresponding to 0.058 AU. Looking at the relative probabilities of the various orbital solutions, it appears that there is only about a 20% chance that TRAPPIST-1d orbits inside the conservatively defined HZ for a synchronously rotating, Earth-size planet. Obviously more data, such as that Kepler is schedule to collect, will help to resolve the situation with this planet but there appears to be only a fairly slim possibility that this extrasolar planet is potentially habitable given what we know today.
Conclusions
The discovery of three Earth-size extrasolar planets orbiting a nearby ultracool dwarf star is certainly important and suggests that such worlds may be common around these tiny stars. And given the large size of these planets in comparison to the stars they orbit, it opens the unique opportunity to study the atmospheres of such worlds using current and future instruments. But contrary to the claims being made in the media, it appears unlikely that any of the three planets discovered orbiting TRAPPIST-1 are potentially habitable in the Earth-like sense. The high effective stellar fluxes of the inner two planets make it more likely that they are Venus-like than Earth-like. The situation with the outer planet is ambiguous and will require more data to define its orbit better but at this time it seems probable that it orbits beyond the outer edge of the HZ.
But whether these extrasolar planets are actually habitable or not, their continued study should prove to be vital in characterizing the conditions of similar such worlds orbiting dim stars and provide important insights into properties of extrasolar planets in general as well as determine the actual limits of planetary habitability.
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Related Reading
“Habitable Planet Reality Check: Terrestrial Planet Size Limit”, Drew Ex Machina, July 24, 2014 [Post]
“Architecture of M-Dwarf Planetary Systems”, Drew Ex Machina, October 24, 2014 [Post]
General References
Michaël Gillon et al., “Temperate Earth-sized Planets Transiting a Nearby Ultracool Dwarf Star”, Nature, Vol. 533, pp. 221-224, May 2, 2016 [Preprint]
R. K. Kopparapu et al., “Habitable zones around main-sequence stars: new estimates”, The Astrophysical Journal, Vol. 765, No. 2, Article ID. 131, March 10, 2013
Ravi Kumar Kopparapu et al., “Habitable zones around main-sequence stars: dependence on planetary mass”, The Astrophysical Journal Letters, Vol. 787, No. 2, Article ID. L29, June 1, 2014
Leslie A. Rogers, “Most 1.6 Earth-Radius Planets are not Rocky”, The Astrophysical Journal, Vol. 801, No. 1, Article id. 41, March 2015
Jun Yang et al., “Strong Dependence of the Inner Edge of the Habitable Zone on Planetary Rotation Rate”, The Astrophysical Journal Letters, Vol. 787, No. 1, Article id. L2, May 2014
“Three Potentially Habitable Worlds Found Around Nearby Ultracool Dwarf Star”, ESO Science Release 1615, May 2, 2016 [Press Release]
“Promising Worlds Found Around Nearby Ultra-cool Dwarf Star”, NASA Press Release, May 2, 2016 [Press Release]
It might be worse than that, once the short rotation periods are also considered. See Kopparapu et al. arXiv:1602.05176 [astro-ph.EP] which indicates the Yang et al. (2014) estimates for the inner HZ boundary are too close to the star because their models forced the simulation to be in a slow-rotation regime.
Always we talk about tidal locked planets it comes to my mind that we just do not know how climate would behave under a permanent dayside and nightside planet, i mean it says “both of these worlds have likely lost all of their water and have Venus-like surface conditions” because of the high stellar flux, however we are talking about a tidal locked planet, isn’t it possible to the water that boils on the dayside travel to the ‘terminus’ zone or whatever the temperatures are right for liquid water on the nightside and become liquid again? Creating an eternal water cycle that way.
Even with such a high stellar flux i believe at least the nightside or part of it will have habitable temperatures, assuming the atmosphere isn’t too thick to lead the heat all the way to the nightside maintaining high tempreatures. That being said it seems possible that TRAPPIST-1c may be habitable on specific surface sites even if they are on the nightside. It is said the flux can go up to 1.47 in the case of a tidal locked planet, i am not so sure, when i think about Venus blackbody tempreature depending on how thin the atmosphere there would be it seems to me at least the poles would have the right tempreatures.
The same aplies to TRAPPIST-1d, since probably there is a permanent dayside, it is quite possible there may be at least a small patch of habitable conditions on the center of the side facing the star. However the nightside there could be extremely cold which may lead to a unstable climate (locked frozen atmosphere on the nightside?).
I wonder what realistic simulations of tidal locked planets have to say about that assuming different atmospheric pressures and CO2 levels.
There is a huge body of peer-reviewed literature published over the last two decades concerning various detailed climate models exploring the potential habitability of synchronously rotating planets. And even they will suffer a runaway greenhouse effect and irreversible water loss once their stellar fluxes exceed a certain value – in the case of TRAPPIST-1, that is around 1.47 times that of the Earth. All I can do is suggest that you read some of the literature such as Yang et al. I cite in the references (and the references in their work). I would also recommend reading “How to Find a Habitable Planet” by James Kasting – the “father” of our modern understanding of Earth-like planetary habitability. It is a relatively easy read and still available, as far as I know. I would especially recommend Chapter 6 in that book, “Runaway Greenhouses and the Evolution of Venus’ Atmosphere”.