More recently, we commented on the discovery of Proxima b, a planet that has become a cherry on top of an exoplanetary cake. And on February 22, 2017, with fanfare, it was announced the discovery of three planets at once in the habitable zone of another red dwarf, TRAPPIST-1. This system is nearly ten times farther than Proxima Centauri, but there are at least two circumstances that make the find the second cherry on the cake in the past few months. It:
- there are three planets in the habitable zone at once, this increases the likelihood that at least one of them is suitable for life;
- these planets, in contrast to Proxima b, are transitory, that is, they pass through the star's disk for an earthly observer, which greatly facilitates the observation of their atmospheres.
A few words about the history of the sensation. The system was discovered in 2015 by the small Belgian telescope TRAPPIST. The name - Transiting Planets and Planetesimals Small Telescope South - is tailored to the Belgian beer brand. The telescope is located in Chile at the La Silla Observatory of the European Southern Observatory.
With its help, three transit planets were discovered near the cold red dwarf 2MASS J23062928-0502285 [1], which received the second, more human name TRAPPIST-1 - this was the first planetary system discovered by this telescope. Then the system was observed by the European VLT (Very Large Telescope) telescope, and finally, thanks to data from the NASA Spitzer infrared space telescope, the system was "untangled" and found that there are seven planets. Actually, the last step was the NASA press conference on February 22.
Figure: 1. Light curve of the TRAPPIST-1 star during the 20-day session of the Spitzer space telescope. Green dots - observations with ground-based telescopes. Vertical - the luminosity of the star at the moment in relation to the average luminosity. The diamonds mark the transits of specific planets. The upward ejections of points are most likely stellar flares. There is only one transit of planet h. Its period and orbital radius are estimated from the duration of a single transit (see Fig. 2)
Figure: 2. Light curves of the star during the transits of each of the seven planets
Promotional video:
The habitable zone includes the planets e, f, g, although at first glance planet d is more suitable for heating intensity than g. This requires a rather complex discussion with estimates of the possible greenhouse effect, including a lot of uncertainties. Of course, the concept of a habitable zone is very arbitrary.
No matter how we define the habitable zone, there are serious problems with the real suitability for life of each of these planets. Same problems as Proxima b. They are associated with the nature of red dwarfs.
1. These are stars with very violent magnetic activity. They have a thick convective layer. Unlike the Sun, where heat is transferred outside mainly by diffusion of photons, convection prevails there. The sun also has convection, which is why spots, flares, prominences appear, and on Earth - magnetic storms and auroras. There all these phenomena are much more intense.
2. The luminosity of these stars at the beginning of their biography changes greatly. For the first millions of years, they shine tens or even hundreds of times brighter than in the steady state.
3. The habitable zone of red dwarfs is so close to the star that the planets fall into a tidal closure: either they are always facing the star with one side, or their day is longer than their year (for the TRAPPIST-1 system, the first option is more likely).
What to do, nature for the second time in less than a year slips us just such not very encouraging planetary systems. This is not surprising - they are much easier to find by the spectrometric method (it is impossible to detect the Earth near the Sun in this way), they are more likely to turn out to be transitory, and the transits are more contrasting, finally, there are more red dwarfs than yellow and orange ones.
Figure: 3. Simultaneous transit of three planets. Light curve taken on December 11, 2015 with the European telescope VLT
So, the data on the TRAPPIST-1 system found (we do not present errors).
| Planet | Orbit radius | Period | Planet radius | Heating intensity (in terrestrial units) |
| b | AU 0.011 | 1.51 days | 1.09 Re | 4.25 |
| c | 0.015 | 2.42 | 1.06 | 2.27 |
| d | 0.021 | 4.05 | 0.77 | 1.14 |
| e | 0.028 | 6.10 | 0.92 | 0.66 |
| f | 0.037 | 9.21 | 1.04 | 0.38 |
| g | 0.045 | 12.35 | 1.13 | 0.26 |
| h | 0.063 | ~ 20 | 0.75 | 0.13 |
Star. Mass - 0.08 solar, radius -0.117 solar, luminosity - 0.5103 solar, temperature 2550K
It was possible to roughly estimate the masses of the planets - because of their interaction, the transits are slightly shifted in time. Errors in determining the mass are great, but we can already conclude that the density of the planets corresponds to the rock filling.
Of course, earth-like planets near sun-like stars will be found in the foreseeable future. Actually, several such planets have already been found in the Kepler data, only they are very far away. It is enough to observe several hundred bright stars across the sky (which is planned in the coming years), and such planets will be discovered within a hundred light years (and if you are lucky, even closer).
In fact, comfortable planets near comfortable stars are within 15–20 light years (this follows from the statistics obtained by Kepler), but in order to discover them, space interferometers are needed, which will not appear soon (see [2]).
The hope that at least one of the planets is suitable for life remains. They could initially have a lot of water - they could not form where they are now, and had to migrate to the star from the periphery of the protoplanetary disk - because of the snow line, where there are many ice bodies. True, they migrated back in the era when the star was much brighter. But estimates made for Proxima b show that the hydrospheres of planets could survive a scorching heat of tens of millions of years.
Tidal closure is not fatal if the planet has a thick atmosphere and a global ocean - then heat transfer is able to smooth out the temperature difference between the day and night hemispheres.
A more serious problem is the blowing away of the atmosphere by stellar wind and hard radiation. At the press conference, it was said that the star is calm now. This is true if we mean thermal radiation, but not X-rays: TRAPPIST-1 - measured directly by the XMM space observatory - emits about the same amount of X-rays as the Sun. Since the planets are ten times closer to the star than the Earth is to the Sun, their X-ray radiation is three orders of magnitude higher than that of the Earth.
X-rays do not pose a direct threat to life - they are absorbed by the atmosphere. The problem is in the dehydration of the planet: X-rays and hard ultraviolet light break up water molecules - hydrogen easily evaporates, oxygen binds. Even worse, since there is intense X-ray, there must be an intense stellar wind - it strips off the outer layers of the atmosphere. The only salvation in this case is the planet's magnetic field. Whether these planets have a strong enough field is a question. Maybe there is.
So, the hope remains that some of the planets of the TRAPPIST-1 system are suitable for life. Can this hope be confirmed or denied? It is possible, and much easier than for the case of Proxima b, in which one must observe either the reflected or the planet's own thermal radiation.
It is very difficult to separate it from the radiation of the star. Here, the atmospheres of the planets can be observed in the light, which is incomparably easier.
In the case of Proxima b, the new James Webb space telescope will be able to show something only in the extreme case: one hemisphere is hot, the other is frozen. In the case of TRAPPIST-1, it is realistic to see absorption lines in the atmospheres of planets. Or put some restrictions on top. One such limitation has already been set: the inner planets do not have thick hydrogen atmospheres.
Figure: 4. Diagram of the orbits of the TRAPPIST-1 system. The habitable zone is marked in gray. Dotted circles - it is in a slightly different interpretation
Is there a theoretical possibility that James Webb will discover life on one of these planets? The most eloquent marker of life is oxygen. It is fully detectable both as ozone and as O2. Another thing is that a certain amount of oxygen can be formed, for example, due to the dissociation of water molecules by the hard radiation of a star. Estimating how much oxygen is a reliable marker is not easy. It is necessary to know the rate of dissociation and the rate of oxygen binding - there are many uncertainties. But if there is as much oxygen as there is on Earth, there is nowhere to go: only life can give this. If there is little oxygen, this does not mean that there is no life: there was little oxygen on Earth for the first couple of billion years of life.
In conclusion, I would like to express my regret that Russia bypassed the study of exoplanets. There are individuals and individual jobs, but nothing more. But this area does not require gigantic installations - rather, gray matter and perseverance than our science has always been able to boast. Some hope is given by the Russian project Millimetron - a cryogenic space telescope with a 10-meter mirror: in the project, the study of exoplanets is one of the first points. However, this is a topic for a separate publication.
Boris Stern, astrophysicist, Ph. D. physical -mat. sciences, led. scientific. sotr. Institute for Nuclear Research RAS (Troitsk)