Very little is known about the TRAPPIST-1 planets, named b through h in order of distance from their sun. They can't be seen directly with our technology. Instead, astronomers discovered them by measuring tiny blips in the light emanating from their star as the planets crossed in front of it - a phenomenon called "transiting". By studying the frequency of those transits, the scientists could figure out the length of the planets' orbits and their distance from the star.
TRAPPIST-1's discoverers also determined that the six inner planets are locked in an orbital resonance, meaning that the lengths of their orbits are related by a ratio of whole numbers. Because of this, the bodies exert regular gravitational influences on one another. By measuring those influences, the astronomers could determine the mass of the planets, something that is impossible to figure out from transiting data alone. That in turn allowed them to loosely calculate the bodies' densities.
With this scant information, Barr Mlinar and her colleagues set about building a model of what those worlds might be like. Having an estimate of each planet's density allowed them to guess what the planets might be made of.
"It's common sense," she said. "Like if your grandmother hands you a box of cake, you can say, 'given the size of this box and how much this cake weighs, this has to be fruitcake.' "
Dense planets like Mercury, Venus and Earth are the fruitcakes of our solar system, composed of heavy iron and silicate rock. On the other end of the spectrum, there are worlds made light by a high proportion of water; for example, Jupiter's moon Ganymede, which is covered in ice. Those same principles apply in TRAPPIST-1, assuming this system behaves in accordance with our simplest scientific models (which, Barr Mlinar admits, is a big assumption).
The scientists also calculated the influence of tidal heating on each of the TRAPPIST worlds. As the planets circle their sun in elliptical orbit, the gravitational push and pull creates friction in their interiors, generating heat. This same phenomenon is responsible for warming the centers of moons like Jupiter's Europa and Saturn's Enceladus in our own solar system.
With this information in hand, the planetary scientists could start to characterise these alien places.
The TRAPPIST-1 system resembles our own, but in miniature. The star at its centre, a red dwarf, is small and cool, though prone to violent flare-ups that send radiation streaming into space. Its planets orbit closely, the most distant finishing its circuit around the sun in just 20 days. This closeness means the planets are probably tidally locked; one side always faces the star, while the other is cloaked in constant darkness.
TRAPPIST-1b, the body closest to the sun, is a surprisingly low-density planet and probably harbours a good dose of water. It's also likely subject to intense tidal heating, which could generate volcanic activity. Barr Mlinar imagined a wet world where undersea volcanoes gush hot gas and molten rock and intense radiation lashes the surface.
Its neighbour, TRAPPIST-1c, is much more dense but similarly warped by tidal forces; this rock and iron-rich body may also boast volcanoes. That bodes poorly for the planet's potential habitability. But it's an exciting prospect to Barr Mlinar, because volcanic eruptions spew huge amounts of material into the atmospheres of their planets - and that material might one day be detectable by telescopes on Earth.
It's common sense. Like if your grandmother hands you a box of cake, you can say, 'given the size of this box and how much this cake weighs, this has to be fruitcake.'
In an early analysis published in 2016, scientists reported that planets b and c have small, contained atmospheres - the kind that envelop Earth, Venus and Mars.
The system's more distant planets, f and g, are chilly places resembling the low-density, ice-covered worlds in our outer solar system, including Saturn's moon Enceladus and Jupiter's moon Ganymede. And planet h is so light it could theoretically be made entirely of ice.
But the best real estate in the TRAPPIST-1 system are planets d and e. These sit in the "Goldilocks zone," neither too close nor too far from their sun, where water can be liquid on their surfaces, and life could theoretically thrive.
Planet d receives enough sunlight that its effective surface temperature (assuming it lacks an atmosphere) is about 18C. Given its small size and low density, it is probably rich with water and may be covered by a global ocean. And since its interior is churned by substantial amounts of tidal heating, it has enough geothermal activity to drive complex chemistry in its ocean - creating conditions similar to Earth's oceans.
The more distant Planet e receives less sunlight, making it about as warm as the South Pole in summer. But of all the TRAPPIST-1 planets, Barr Mlinar said it's the one she'd most like to visit. "I don't even have to think about it!" she declared. A specialist in icy bodies, Barr Mlinar is attracted to this world where the effective surface temperature is close to the melting point of ice.
"There could potentially be liquid water on the surface, there could be places where the ice is going to be really mushy," she said. "That's going to give really interesting ice tectonics."
Barr Mlinar cautioned that these planet descriptions are wholly hypothetical, reliant on paltry data and a heap of assumptions. It will take a lot more research - likely with telescopes that don't even exist - to know what TRAPPIST-1 planets truly look like.
Even so, theorising about the geophysics of these worlds can be useful, she said. For example, the knowledge that TRAPPIST-1c may be volcanically active could guide astronomers' search for molecules in its atmosphere.
Not to mention, it's cool. Seen through astronomers' telescopes, these planets are just "points of light - or not even points of light," she said, "just signals in a light curve, measurements in an instrument."
But apply some basic principles of planetary science, and those bits of data coalesce into physical worlds: active or inert, hostile or appealing. We can start to imagine feet setting down on that alien ground.
As Barr Mlinar said, "It helps everyone in the community to see these planets as places where the types of process that operate in our solar system are also operating."
