The Planets of TRAPPIST-1: Multiple Potentially Habitable Earth-like Worlds in a Single System

Though there’s plenty happening on Earth to write about, considering how grim much of it is, it’s something of a relief to be able to instead comment on some fascinating astronomy news. I’m of course referring to the announcement that seven apparently Earth-like planets have been identified orbiting a single star, and one that is on a cosmic scale quite close (though on a human scale the star and its planets are still very, very far away, a point I will come back to later). To put this in perspective, prior to this discovery, the star with the greatest number of terrestrial (i.e., rocky and relatively similar to Earth in size) planets was our own Sun with four, namely Mercury, Venus, Earth itself, and Mars. There are no doubt other systems with multiple terrestrial planets, but due to the relative difficulty of spotting such planets, most discoveries to date have been of larger planets, though there are a few known systems with one or more super-Earths (planets somewhat larger than Earth which may also be rocky). What makes this newly discovered system even more fascinating is the fact that several of the worlds appear to be in the star’s habitable zone, and the relative closeness of the system and the nature of its parent star means that we will be able to study their possible atmosphere is in the near future, and maybe determine if the gases present in those atmospheres indicate the presence of life.

The star that the newly discovered planets are orbiting is called Trappist-1 (technically it should be capitalized as TRAPPIST-1, but I will use the lower case form) and it is located 39 light years away. Though most of its planets seem to be similar to Earth in mass and radius, the star itself is nothing like the Sun. It is a very cool red dwarf star, which means that it produces far less energy than the Sun. One of the charts used in some articles about Trappist-1 and its planets compared the orbits of the latter not only to those of the four terrestrial planets orbiting the Sun but also to the orbits of the four Galilean satellites of Jupiter. This makes sense when you realize that Trappist-1 is in many ways as similar to Jupiter as it is to the Sun. In fact its diameter is only slightly greater than Jupiter’s, and the orbits of its planets resemble those of the Galilean satellites more than they do those of the four terrestrial planets in our Solar System, though they are still considerably further out from the star than Jupiter’s moons are from Jupiter. The ratio of their masses in comparison to their star is also similar to that of the Galilean moons in comparison to Jupiter. Nevertheless, while Jupiter is just a planet (though a very big one), Trappist-1 is a star powered by nuclear fusion in its core, and it is still more than 80 times as massive as Jupiter, though it is only 8% as massive as the Sun. So it gives off enough energy that many of its planets, given their close orbits, could potentially have liquid water on their surfaces, unlike the moons of Jupiter, which remain frozen on their surfaces (though Europa at least almost certainly has liquid water underneath its icy crust).

The planets, designated b to h in order of their distance from the star, orbit at distances that are a fraction of Mercury’s distance from the Sun. They were discovered by the transit method, which involves observing the dimming of the star’s light as a planet passes in front of it. Trappist-1b orbits the star in just 1.5 Earth days, and even the most distant planet, Trappist-1h, takes only about 20 days to complete on orbit (though this planet is the one about which there is the greatest uncertainty). Though at this point we only have rough estimates of the planets’ masses and diameters, it appears that c and g are somewhat larger than Earth but not by very much, and the other planets are slightly smaller than Earth, though even the smallest two, d and h, are larger than Mars. The planets are likely to be tidally locked so that they always show the same face to their star, just as the Moon does toward Earth. However, if they have atmospheres the temperature contrast may not be as great as it would be otherwise.

The planets d, e and f receive similar amounts of energy from Trappist-1 as the Earth does from the Sun, putting them in what is known as the habitable zone. Some of the other planets could potentially have moderate temperatures and liquid water, depending on their particular circumstances (for instance, if g has a thick enough atmosphere it might retain enough heat to stay above water’s freezing point). However, there is a lot of uncertainty involved. If Earth didn’t have an atmosphere with a greenhouse effect, it would be frozen despite the amount of energy it receives from the Sun (of course our problem now is that we are increasing the amount of greenhouse gases, which could disrupt the climate’s equilibrium in the other direction). It is uncertain how many of these planets have atmospheres or how thick they might be. Red dwarf stars are prone to violent flares, which may strip atmospheres from close in planets. The planets closer in may also have lost all their water or have undergone a runaway greenhouse effect like that on Venus in our solar system. The ones further out may have ended up like Mars if their atmospheres are too thin.

Despite these caveats, given the number of planets, it seems likely that at least one or two have moderate, Earth-like temperatures and high potential for liquid water on their surfaces. What’s more, since these planets are much closer to us than many other exoplanets (planets orbiting stars other than the Sun) that are in their star’s habitable zones – for example, the probable super-Earth Kepler-452b, which orbits a Sun-like star, is about 1400 light years away – and they orbit close to a dim star and transit it frequently, they are much easier to study. With slightly improved telescopes such as the ones that should be coming online in the next few years, it should be possible to analyze the components of these planets’ possible atmospheres and determine whether gases commonly produced by living things are present. There are a few other potentially habitable exoplanets that are closer these ones, notably the one orbiting the closest star other than the Sun, Proxima Centauri (or Alpha Centauri C), but not all of these transit their stars from the perspective of Earth (for example, Proxima Centauri b, which was discovered by the radial velocity method which measures changes in a star’s apparent velocity due to the gravitational effects of a planet, apparently does not transit its star), and most are super-Earths, rather than being truly Earth-like. What’s more, the presence of multiple Earth-like planets in the habitable zone of the same system makes it a uniquely attractive target for study.

Of course, just because the planets are potentially habitable does not mean that they are actually inhabited by any form of life. As noted, they may be in fact either too hot or too cold, too lacking in water, or too heavily irradiated, for life to have developed. We still don’t know what conditions are required for life to appear, though from what we see on Earth we know that life is amazingly tenacious and adaptable once it does appear. Furthermore, even if one or more of the planets has some form of life, the chances that they have intelligent life are much smaller, and the chances of a technological civilization like that of humanity are even smaller. As I have argued before, I suspect that the real reason we’ve seen no signs of advanced alien civilizations all over the galaxy is that while life itself may turn out to be common, multicellular life (which only appeared on Earth billions of years after single-celled life forms) is much rarer, and intelligent life that happens to evolved the physical characteristics for building a civilization (a condition that prevents, say, dolphins from making tools or building spaceships) and lives on a planet with the right resources (a lack of iron on the planet’s surface, for instance, would make it hard for even human-like creatures to get very far towards developing advance technology) may be so rare that it only exists on a few planets out of all the billions in the entire galaxy at any given time. But even evidence of “primitive” life would be an incredibly exciting discovery, and these planets give us the best chance of discovering it outside our solar system in the near future that we have yet seen.

Perhaps inevitably, a number of articles about this discovery mentioned jokingly the possibility of escaping the growing mess created by the new US administration and the threat of right-wing populism in Europe by colonizing these potentially habitable planets, or alternately sending all the troublemakers on Earth to them and thus ridding ourselves of them. Unfortunately, direct exploration of these planets, even by robotic spacecraft, remains an extremely distant prospect. As I explained in my commentary on the discovery of Proxima Centauri b, we are a long way from being able to travel to other stars in a reasonable time frame. One article on the Trappist-1 planets noted that one of the fastest spacecraft ever launched, the New Horizons probe that explored Pluto (reaching that distant planet in a little less than 10 years after its launch from Earth), would take about 750,000 years to reach Trappist-1. It’s possible that an effort like Breakthrough Starshot might actually see miniature spacecraft traveling to the closest star systems, such as Alpha Centauri, before this century is over, but even if that ambitious project succeeds, it would still take a couple of centuries for spacecraft traveling at the speeds targeted by the project to reach Trappist-1, which is almost 10 times as distant as Alpha Centauri. Nevertheless, simply by studying the planets from Earth, we may be able to discover if any of them host life. If we do find solid evidence of life on any of them (which, it must be emphasized again, is not guaranteed), it may provide the motivation for even more intense efforts at finding better methods of starship propulsion. Even the knowledge that life exists on a planet or, even more excitingly, multiple planets orbiting a nearby star would have a dramatic effect on our view of our place in the universe.