What the Team Discovered
Most extrasolar planets (planets orbiting other stars) are found via transits. If we lie near the orbital plane of an extrasolar planet, then once each orbit the planet will transit, or pass in front of its star. As the planet transits, it will block a small portion of the star’s light. If astronomers accurately measure the star’s brightness during a transit, they can detect the planet’s existence. Detailed study of the data will reveal the planet’s diameter.
In 2015, a research team discovered three earth-sized planets orbiting this star, using the robotic TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) observatory in Chile (obviously the otherwise faint, nondescript star that hosts the seven planets derives its name from this telescope, though the star also goes by the name 2MASS J23062929-0502285). Follow-up observations using TRAPPIST, other ground-based telescopes, and the Spitzer Space telescope resulted in the discovery of four additional planets and constrained the properties of all seven planets. Extrasolar planets are named by appending lower case Latin letters to the names of the stars they orbit, starting with the letter b, and continuing sequentially in order of discovery. In the case of the seven planets orbiting TRAPPIST-1, the planets happened to have been discovered in order of increasing distance from the star. Therefore, the innermost planet is TRAPPIST-1 b, and the outermost planet is TRAPPIST-1 h. Henceforth, I shall refer to these planets by their letters only.
In the Zone
All seven planets are similar in size to the earth. The smallest, h, is 75.5% the earth’s diameter, while the largest, g, is 112.7% the earth’s diameter. Having earth-like size is important in looking for planets where life might exist. If a planet is too small (as Mercury is), it will lack sufficient gravity to hold on to an atmosphere. On the other hand, if a planet is too large (as Jupiter is), its gravity will be too strong, and it will hold on to the wrong gases. Therefore, all seven of these planets look promising. However, having the proper size does not guarantee an atmosphere proper for life.
Furthermore, being the right size is not sufficient in itself. For instance, if earth were much closer to the sun, the earth would be far too hot for liquid water to exist. But if the earth were much farther from the sun, it would be too cold for liquid water to exist. Scientists can compute the region around a star (the habitable zone) where liquid water might exist on a planet’s surface. However, this computation relies upon assuming an atmosphere like the earth’s. If a planet’s atmosphere is significantly different, the actual habitable zone would be different, and there may not be any habitable zone at all. What are the atmospheres like on these seven planets? We don’t know, but we can make some estimates.
Those estimates depend upon the compositions of the planets, but we don’t know those either. However, if we know a planet’s mass and size, we can determine its density, from which we can infer its composition. Transit observations normally do not reveal an extrasolar planet’s mass, but in this case, there is a subtle effect that we can use to infer mass. The six innermost planets have orbital resonances. This means that the ratios of their orbital periods are ratios of integers. For instance, each time planet b orbits eight times, planet c orbits five times. Each time planet c orbits five times, planet d orbits thrice. The resonances probably are the result of gravitational interactions.
Furthermore, the observations showed variations in the timings of the transits (TTVs, for Transit Timing Variations). The TTVs undoubtedly are due to further gravitational interactions between the planets, which allow us to estimate the masses of the six innermost planets. These masses are far more uncertain than the diameters of the planets. Combining the diameters and inferred masses, we can estimate the densities of the six innermost planets. The densities are close to that of earth. The least dense, planet f, has density 60% of earth, while the densest, planet c, has density 117% that of the earth. These densities are similar to the densities of the terrestrial planets, the four innermost planets of our solar system. However, keep in mind that there is much uncertainty in these values, and consequently it is possible that at least four of the six innermost orbiting planets could have Jovian (the outer four planets of our solar system) composition. If those planets have Jovian composition, it is extremely unlikely that they have conditions like that on earth.
It is premature to conclude that any of these planets, despite being in the habitable zone, are conducive for life.
The team found that three of the seven planets, e, f, and g, lie in the habitable zone of their star. Planet f has the least uncertainty in its density, but it also has the lowest density, 60% of earth. It is not clear if this planet has enough gravity to maintain an atmosphere proper for life. The masses and densities of the other two planets have much higher uncertainties, so it is unclear whether they have the proper composition. Indeed, while news accounts did not report this, the authors of the study were aware of these caveats. They concluded that the six planets probably formed farther from the star and then migrated inward. Astronomers expect that planets that form far from a star will be rich in volatiles, the very light elements (as the Jovian planets are). The authors of the paper wrote:
The planets’ compositions should reflect their formation zone, so this scenario predicts that the planets should be volatile-rich and have lower densities than Earth, in good agreement with our preliminary result for planet f.
Despite this caveat, the researchers further wrote:
We deduce that planets e, f, and g could harbour water oceans on their surfaces, assuming Earth-like atmospheres.
However, it is very doubtful that planet f has an earth-like atmosphere, and it is doubtful that the other two do as well. Therefore, it is premature to conclude that any of these planets, despite being in the habitable zone, are conducive for life.
Also not reported in news accounts is that the research team expects that all seven planets are tidally synchronized. This means the planets probably rotate and revolve at the same rate so that each planet keeps one side facing the star as they orbit. This is due to tides raised on the planets by the star. A similar thing happens with natural satellites, or moons, that orbit planets in the solar system. Clearly, this is not a good situation. The temperatures on the daysides of these planets probably are very hot, while night side temperatures are very cold. Life probably would be possible only in a relatively narrow zone in between on each planet. However, it is probable that extreme conditions, such as high winds, prevail in these regions as well.
Left unsaid in many of these sorts of studies announcing discoveries of earth-like planets is a discussion of what types of stars these planets orbit, and this latest announcement is no exception. Most of the supposed earth-like planets orbit M dwarfs. M-dwarf stars frequently are variable stars, meaning that their light varies. The sun appears to be very stable, but with all the concern about climate change, it ought to be obvious that even modest changes in a star’s luminosity would be detrimental to life on an orbiting planet. More concerning is the manner in which many M-dwarfs vary. Often it is through very energetic flares from their surfaces. The sun flares occasionally, but flares on M-dwarfs can be far more powerful and numerous than solar flares. Most supposed earth-like planets orbit far more closely to their stars than earth orbits the sun. Consequently, a flare on an M-dwarf would have far more impact on any close planets orbiting it. Consider a flare on TRAPPIST-1 that is comparable to a solar flare and is directed toward one of the planets. Due to being much closer to their star, the effects would be 500–1,000 times greater than a similar solar flare directed toward the earth. I was not able to find any information on the variable nature of TRAPPIST-1, so I am not sure whether this star is a variable. Given its extreme faintness, we probably do not yet know whether TRAPPIST-1 is a variable star.
There have been many extrasolar planets touted as being “earth-like.” As always is the case, the details indicate that the conclusion is far less certain than most people realize. This is the case of the most recent announcement of three earth-like planets orbiting TRAPPIST-1. Several assumptions must be true for any of these planets truly to be earth-like, and there are reasons to believe that, like the others, there is far less here than thought. As far as we know for certain, there are no earth-like planets.
There was a very interesting result in the accompanying material following the paper. Given the gravitational interactions between the planets, the research team studied the long-term stability of the planetary system. Numerous simulations revealed that the system likely would disrupt within a half-million years. A different analysis indicated a 25% chance of the planetary system undergoing disruption within a million years, and only an 8.1% chance of surviving a billion years. According to the evolutionary timeframe, it took three billion years for higher life to develop on earth, so these results are a serious problem for the possibility of life on the planets orbiting TRAPPIST-1. Not to worry—the authors of the study concluded that these calculations must be wrong, for they wrote:
The system clearly exists, and it is unlikely that we are observing it just before its catastrophic disruption, so it is probably stable over a long timescale.
Or perhaps it is evidence that this system is not very old at all.