Seven Earth-sized planets discovered orbiting a nearby star

An international team of astronomers has found that a nearby star is accompanied by a swarm of at least seven small, rocky worlds.

One of the eyecatching claims in the work, published today in Nature, is that in the appropriate circumstances, there is a chance that any (or all) the planets could host the right conditions for liquid water to exist on their surface.

The orbits of several of the planets also appear locked in a delicate dance that could explain how such planetary systems form.

The host of this newly uncovered planetary system is a dim little object called TRAPPIST-1, which lies almost 40 light years from our Sun.

TRAPPIST-1 is so tiny that it only barely counts as a star. It has just 8% of the mass of the Sun and lies right on the boundary between normal stars and brown dwarfs.

Were this tiny star slightly less massive, it would be too lightweight to fire nuclear fusion in its core, and it would be a failed star.

If TRAPPIST-1 was dropped into our Solar System in the place of the Sun, it would shine in our sky only marginally more brightly than the full Moon. It would be deep red in colour, with a surface temperature of just over 2,200℃.

In this hypothetical scenario, the Solar System would be a lifeless place. Earth’s oceans would freeze, and then our atmosphere would do the same.

To be warm enough to potentially host liquid water, any planets around this dimly glowing ember would have to huddle close and tight.

Three little worlds

In May last year, a team of astronomers using the TRansiting Planets and PlanetesImals Small Telescope (TRAPPIST) announced their first exoplanet discovery.

That team had found striking evidence that a dim, ultracool dwarf star was host to at least three small rocky planets. Over a period of a 62 days, the star was observed to wink periodically, dimming slightly, then returning to its normal brightness.

These winks were the telltale sign of planetary companions.

As this was the telescope’s first discovery, the star was named TRAPPIST-1, and the planets TRAPPIST-1b, c and d.

The inner two planets were found to orbit the star every 1.5 and 2.4 days, respectively. They were small, only very slightly larger than Earth.

Because they huddled in so close to the star (within 2 million km) they would be warm. Most likely, they would be too warm to host liquid water on their surface. But if conditions were just right, then there might, just might, be an outside chance that they could be both warm and wet.

The seven new ‘Earths’

With today’s announcement, the TRAPPIST-1 system has suddenly became much more interesting. The same team has invested a huge amount of time following up on their discovery, using telescopes across the planet.

Those observations have borne great fruit, with the capture of a large sample of transits. The result is the confirmation of the three previously known planets and the discovery of four more.

The TRAPPIST-1 planetary system is amazingly compact, with all the planets orbiting within 10 million km of their host star. To put that into perspective, Earth orbits almost 150 million km from the Sun.

So the scale of the TRAPPIST-1 system is dramatically different to that of our Solar system. But a better analogue lies close at hand – the satellites of Jupiter.

A Galilean solar system?

The scale of the TRAPPIST-1 system is strikingly similar to that of the Jovian system. Jupiter’s four most massive moons – Io, Europa, Ganymede and Callisto – all cluster within 2 million km of the giant planet. Known as the Galilean satellites, their orbital periods span the same relative range as TRAPPIST-1’s planets.

The inner three Galilean satellites are trapped in what is known as a Laplace resonance, with Io completing four laps of Jupiter in the time it takes Europa to complete two, and Ganymede to complete one.

Their tightly packed nature and orbital resonance gives an important clue as to their formation. The idea is that those moons formed further from Jupiter than their current orbits, and migrated inwards, becoming trapped in the resonance as they went.

Which brings us back to TRAPPIST-1’s planetary system. Where three of Jupiter’s moons lie trapped in mutual resonance, the TRAPPIST-1 system takes things to a whole new level. The six inner planets all lie in near-resonance, with periods in the ratio 24:15:9:6:4:3.

This is the first time such a long near-resonant chain of orbits has ever been found. It remains possible that, when more data are available for the outermost planet, it will add to the chain.

This striking similarity to the Galilean satellites suggests a similar formation process. It hints that the planets orbiting TRAPPIST-1 may have begun their formation further from the star, before migrating inwards to their current orbits.

Could there be water?

Based on their distances from TRAPPIST-1, the authors make a first rough attempt to calculate whether the planets could host liquid water.

The innermost should be too warm (unless it is highly reflective, absorbing little of the light that falls upon it). The outermost should be too cold (unless it has a thick, insulating atmosphere).

For the others, things seem a little more promising. In terms of the amount of energy they receive from their star, planets c, d and f seem strikingly similar to Venus, Earth and Mars.

In the Solar system, it seems likely that Venus was once warm and wet, before a runaway greenhouse led to the ocean’s boiling and the planet becoming inhospitable.

Mars was once warm and wet before its atmosphere bled to space and was chemically drawn down into the surface, causing the planet to become frigid and barren.

The idea that TRAPPIST-1’s planets might have migrated from further out in the system could add weight to the possibility of their having water.

Farther from TRAPPIST-1, temperatures are colder, which would mean that in the nebula from which the planets formed, there should have been plenty of water ice for the planets to acquire as they accreted, giving the possibility that these are wet worlds.

But there’s a great difference between “maybe” and “yes”. As it stands, we have no evidence that these planets are wet. They could just as easily be barren, airless worlds, stripped of their atmospheres by TRAPPIST-1’s youthful exuberance.

Or, given their resonant nature, they could be volcanic hellholes, just like Jupiter’s Io, with continual and fiery volcanic activity, driven by tidal interactions with one another. We simply don’t know. But that could change in the coming years with new and better observations.

Until then, we can only speculate on what the planets around this ultracool, dim ember might be like.