Less than a year ago, an international team of astronomers put the star TRAPPIST-1 in the news. TRAPPIST-1 itself is unremarkable, belonging to the class of small, dim stars known as "ultracool dwarfs." Rather, it was the presence of three planets orbiting the star that made the news. While they probably orbit too close to support liquid water, TRAPPIST-1's proximity to Earth—it's less than 40 light years away—makes detailed observations of any planetary atmospheres a realistic possibility.

But the orbit of the outermost planet, TRAPPIST-1d, wasn't well defined by the initial observations, causing its discoverers to go back for some followup observations. Those turned up four more planets, three of which are likely to be in the habitable zone of their host star. As the orbit of the outer one is, once again, uncertain, more observations will undoubtedly be in the works. Who knows what they'll turn up?

The TRAPPIST family

TRAPPIST-1 got its name from the telescope that first spotted the planets, the Transiting Planets and Planetesimals Small Telescope, although researcher Emmanuël Jehin noted, "By the way, Trappist is the name of a famous Belgian special beer." It's a Belgian project that searches nearby dwarf stars for the presence of planets, since the dim star would make observations of planetary atmospheres much easier. The team's first telescope is based in Chile's Atacama desert, but it has since added a second in Morocco.

Both telescopes were used to try to sort out the orbit of TRAPPIST-1d, which was thought to be the outermost planet in the system. Other instruments joined in the observations, including the 3.8-meter UK InfraRed Telescope (UKIRT) in Hawaii, the 4-meter William Herschel and the 2-meter Liverpool telescopes at La Palma, Spain, and the South African Astronomical

Observatory 1.0-meter telescope. Unfortunately, the additional data only confused matters: "these light curves showed several transit-like signals of unknown origins that we could not relate to a single [orbital] period."

So, they brought in the big guns: NASA's Spitzer space telescope. The initial results were so intriguing that Spitzer provided 20 days of near-constant imaging of TRAPPIST-1. This imaging identified 34 individual transit events; combined with ground based data, it was possible to clearly identify six orbiting planets. (Two of the transits previously thought to be TRAPPIST-1d turned out to be from different planets entirely; another turned out to be three planets transiting simultaneously.) A seventh was only observed a single time, so further observations will undoubtedly follow; our catalog of planets surrounding TRAPPIST-1 may not be complete yet.

The six inner planets, TRAPPIST-1b-g, are tightly packed around their host star, with orbital periods of 1.51, 2.42, 4.04, 6.06, 9.1, and 12.35 days. That means they also orbit close enough to exert gravitational influences on each other, which may speed up or slow down individual orbits. These create what are called transit timing variations, where a planet may appear in front of its host star a few minutes earlier or later than would be expected if it is orbiting on its own.

These orbital interactions may have been key to stabilizing the TRAPPIST-1 system. The ratios of the time it takes for neighboring planets to orbit are all ratios of integers: 8/5, 5/3, 3/2, 3/2, and 4/3. Integer ratios provide interactions that help prevent the sort of planetary chaos that can launch planets out of the exosolar system or send them spinning into the host star. When we see resonances like this, it's taken as a sign that the planets formed farther from their host star and migrated inward due to friction from the disk of material they formed within. Orbital resonances check this inward migration and produce tightly packed systems like TRAPPIST-1.

Rocky, maybe Earth-like?

The orbital interactions tell us a lot more than the planets' history, though. Since the transit timing variations depend on gravitational interactions, and gravitational attraction depends on mass, the data provides some constraint on the mass of the planets. While the current numbers aren't very exact, the precision will go up with further measurements. Most of them have masses similar to Earth's with a high of 1.4 times Earth, and a low of 0.4 times. The amount of starlight blocked during their transit provides a measure of the planets' size. Their radii range from 75 percent of Earth's size up to 113 percent.

On their own, these numbers suggest Earth-like proportions. Combined, they give us the density, which hints at their composition. The six inner ones appear likely to be rocky with the exception of planet f, which has a lower density that may reflect a lot of volatiles—either a larger atmosphere or a more water-rich composition.

But a key question is what state that water is in: ice, liquid, or vapor? All of the six well-characterized inner planets orbit at distances far shorter than Mercury orbits the Sun. If TRAPPIST-1 were the Sun, all of them would be baked. But it's only eight percent of the Sun's mass, and it burns hydrogen very slowly. As a result, there is a sharp drop-off in the amount of radiation reaching the planets from TRAPPIST-1. Even the closest gets a bit over four times the amount of light that Earth receives; the most distant receives only 0.13 times. Or, as the authors put it, "planets c, d, and f have stellar irradiations very close to those of Venus, Earth, and Mars, respectively."

As a result, every single one of the six inner planets is within what's called the temperate zone, where the right conditions mean that water could exist on the surface. Climate models, however, suggest that the three inner planets would end up in a runaway greenhouse warming. By contrast, TRAPPIST-1e, f, and g could all have oceans on their surface. Of course, the planets are almost certainly tidally locked, keeping a single face turned toward their host star. This may turn them into what has been termed an "eyeball Earth" (see sidebar).

The other big factor to consider when it comes to habitability is the geology of these planets. Not only are some of them big enough to potentially have sustained plate tectonics, but their close proximity may allow gravitational stresses to warm their interiors through tidal forces, much as happens with Jupiter's large moons. So, there's a chance that these planets are being heated both from within and without. Active geology can also have a large influence on the composition of a planet's atmosphere.

We may not have long to wait to start finding out. Remember, TRAPPIST-1 was chosen for observations because it's a dim star that makes observations of planets easier. "Maybe the most exciting thing here is that these seven planets are very well-suited for detailed atmospheric study," said team member Michaël Gillon. Amaury Triaud, another researcher involved in the work, said initial observations are already under way. "We're trying to rule out that the planets have a large hydrogen envelope," Triaud said during a press call. "This is to make sure that the planets are indeed earthlike."

Once launched, the Webb Space Telescope will make these observations even easier, which left Triaud optimistic about future work. "If there is life there," he said, "maybe [we'll know] within a decade."

Nature, 2017. DOI: 10.1038/nature21360 (About DOIs).