It doesn’t take much to get people excited about exoplanets. The idea of a planet orbiting another star is cool enough; bonus points if said planet is inside the star’s habitable zone, where liquid water can form, and if it looks to be the same size as Earth. And if you can find seven such planets, around a single star? Well, then, you’ve hit the jackpot.
The Transiting Planets and Planetesimals Small Telescope (TRAPPIST) survey appears to have discovered just that, astronomers announced in Nature on February 22, 2017. The international team, led by Michaёl Gillon of the University of Liège in Belgium, had spent months confirming their results after detecting three of the planets early last year. Only forty light-years away, the system consists of a dwarf star, dubbed TRAPPIST-1, surrounded by seven Earth-sized bodies. The planets are labeled alphabetically in order of their distance from the star, with the innermost TRAPPIST-1b orbiting in a mere 1.5 days. Three are in the habitable zone, and the relative proximity of the system to ours means that further studies can be conducted to search for signs of life.
Hunting for Other Worlds
That’s not bad, considering the hunt for exoplanets only took off about twenty years ago. In the years since the first exoplanet was discovered, the field has exploded. Some research teams look for radial velocity wobbles in a star caused by the slight tug of an orbiting body; others hunt for the characteristic dimming of a star when a planet crosses in front of it, called a “transit.” Both approaches have led to great success: using the transit approach, the Kepler Space Telescope has increased the count of confirmed exoplanets to over two thousand.
But here’s the catch: out of those discoveries, very few have the potential for life. Part of the reason is that the search is rigged against finding Earth-like planets. A distant observer monitoring the Sun wouldn’t detect Earth using either method: our planet is too small and far away from its star to cause noticeable wobbles, and even if you did notice a dip in light during a transit, you’d have to wait around a while. Since astronomers need at least a few transits to confirm their results, they search for planets with shorter orbits, nowhere near as long as Earth’s one-year sojourn around the Sun.
As a result, most of the planets discovered thus far are Super-Earths hugging their parent star with an affection too hot to be hospitable for life. Or they’re too wet, or not dense enough, or…the list goes on and on.
Given these challenges, it certainly comes as a surprise that the TRAPPIST team could find even one Earth-like planet, let alone seven. But the astronomers were helped by the fact that the system’s parent star is no match for the Sun. A mere M dwarf, TRAPPIST-1 is cool, dim, and tiny, making transits easier to spot. And those transits happen rather frequently, with the furthest planet orbiting in just twenty days. The planets are all closer to their parent star than Mercury is to the Sun, but since the star is ultra-cool, these tight orbits might be just right for life.
An Unusual Case
The system’s compactness is one feature that makes it “like nothing we have seen before,” notes Ed Turner, veteran exoplanet hunter and astrophysicist at Princeton University. In astronomy jargon, the system is “dynamically packed”, meaning there’s no room for another stable orbit. It’s like juggling: you can handle three bowling pins, maybe four if you’re a pro, but try adding a fifth and the whole performance goes awry. Considering how close the planets are to one another, Turner is surprised that the system can even manage to be stable in its current state.
One possible explanation for the system’s seeming stability is the phenomenon of tidal interactions and orbital resonance. In their complex gravitational dance, the planets ended up with orbit periods in nearly integer ratios. For instance, while TRAPPIST-1 e orbits in 4.04 days, its neighbor TRAPPIST-1 f takes 6.06 days, resulting in a 3:2 ratio between their orbits. Such interactions can produce stability by stripping the planets of excessive energy. By a process known as tidal heating, that energy warms up the planets, which could make their weather even balmier than we think.
In addition to occupying such close quarters, the planets are also out of the ordinary in that there are so many of them, all so similarly sized. For some, it is these unusual characteristics—rather than a belief that the discovery could transform the field—that has generated such attention. Michael Lemonick, opinion editor at Scientific American and lecturer at Princeton University, compares the finding to an entry in the Guinness Book of World Records. It’s like “most hops in a minute, or most hops on one leg, or most hops by a bunny,” Lemonick jokes; a system of seven exoplanets might not be revolutionary, but at the very least, it’s an interesting curiosity.
If you were dropped onto the surface of one of these planets, Turner says, “you’d never mistake it for the Earth.”
Lemonick, who chronicled the hunt for our planet’s “twin” in his 2011 book Mirror Earth, is also intrigued by the potential to search the planets’ atmospheres. Because the planets orbit in planes that are very closely aligned with each other and their parent star, scientists can observe several transits directly. During these transits, astronomers can hunt for the tell-tale signatures of life—determining whether the planets are not just inhabitable, but in fact inhabited. The long-awaited James Webb Space Telescope, scheduled to launch next year, will be able to probe the planets’ atmospheres for ozone, methane, and water vapor.
Researchers have their work cut out for them, though. Even if oxygen is found, it will be difficult to tease out its origins; it might be a sign of life, or it might be one of the false positives hypothesized by theorists.
And while the epithet “Earth-like” is one of the most exciting labels one can affix to another world, Turner is wary of using this description. It’s only natural for us Earthlings to use our home planet as the standard for all comparisons, but our environment might not even be the most common setup for life to form. As for the TRAPPIST system, while the planets are Earth-like in terms of size and (potentially) temperature, that’s where the similarities end. If you were dropped onto the surface of one of these planets, Turner says, “you’d never mistake it for the Earth.”
Case in point: because they’re so close to their parent star, the planets are most likely tidally locked, trapped with one side always facing the star. In such a situation, the side basking in light will be much warmer, unless the planet’s atmosphere does a good enough job of spreading the heat around, resulting in incredibly strong winds that put New Jersey weather to shame. But this effect, while possible in theory, has never been observed, and the resulting climate might be unstable.
To be sure, it’s hard not to get excited by the TRAPPIST discovery. The researchers have done their homework. The planets’ gravitational interactions cause slight changes in the timing of their transits, allowing the team could deduce their mass and density. And they’ve already confirmed that the atmospheres of TRAPPIST-1 b and c aren’t dominated by hydrogen, which would have squashed the chances for life.
Even if the system doesn’t harbor life, it’s an excellent specimen for studying M-dwarf stars and their planets. For scientists, it’s the opportunity to find out whether ultra-cool stars such as TRAPPIST-1 make decent parents, or whether their volatile nature is a turn-off. And as for the rest of us? Well, we can enjoy the cool pictures of alien worlds, while pondering what it would be like to always walk in the light, and waiting for the next discovery to blow our minds.