There have been many discoveries of potentially habitable planets orbiting stars other than our own over the last few years. Now things are getting even more exciting. Scientists have documented a star surrounded by no fewer than seven Earth-like planets – several or all of which could be at the right temperature for liquid water, and potentially life, to exist.
But is it possible to know anything about what these planets are like beyond simple measures such as temperature and mass? There are indeed several factors that can give us a clue. So let’s take a look at what planetary processes we might expect to find there – and ultimately whether life could exist.
The seven planets orbit an “ultracool dwarf” about 39 light years away. But don’t think this star is wearing shades. With a mass of only 8% of the sun’s and shining less than 0.1% as brightly, it is at the small, faint end of the “red dwarf” star type, barely able to power itself by nuclear fusion. Proxima, the nearest star beyond the solar system (4.24 light years away), where recently a single planet was discovered, has 12% of the sun’s mass and is an ordinary (not ultracool) red dwarf.
In 2010, a group of scientists began monitoring the closest dwarf stars using a robotic telescope in Chile called TRAPPIST (the Transiting Planets and Planetesimals Small Telescope). They were hoping to find periodic dips in brightness caused by a planet passing in front of the star’s disc, cutting out part of its light (a transit). In 2016, they found their first candidate: an ultracool dwarf.
They named this star TRAPPIST-1 and began to study it with more powerful telescopes, including NASA’s Spitzer space telescope. This fuller survey has now revealed a total of seven transiting exoplanets there (see video).
The amount of light blocked out by each exoplanet during a transit reveals its size. The repeat frequency reveals each exoplanet’s orbital period, from which the laws of gravity allow us to deduce its distance from the star. These exoplanets have no names, but by convention are designated as TRAPPIST-1b (the innermost) to TRAPPIST-1h (the outermost).
Amazingly, the planets of TRAPPIST-1 span only a narrow range of sizes, not much different to Earth. They huddle round their star almost as closely as Jupiter’s major moons to Jupiter, and are all much closer to their star than the Earth is to the sun. However, TRAPPIST-1 is so faint that even its innermost planet may be just about cool enough for liquid water to exist on its surface, while its outermost planet may be just warm enough to avoid global freezing.
Transits reveal exoplanets’ sizes rather than their masses, but mass can be deduced when, as in the TRAPPIST-1 system, there are slight irregularities in transit timings attributable to neighbouring exoplanets perturbing each others’ orbits. This suggests that most of the family are Earth-like in their density and not just their size. There is no way to be sure yet how much water most of them have, if any. Similarly, it’s hard to know whether any resemblance to Earth extends as far as having plate tectonics and a distinction between oceanic and continental crust (like Earth) or a more globally homogeneous crust (like Mars and Venus).
The exception is TRAPPIST-1f which seems to be notably less dense, implying a lot more water and less rock and iron. If this planet has a deep global ocean we can imagine it as one with many hot springs or even volcanoes on its floor. This is because the planets b through g have “resonant orbits” (their periods are simple multiples of each other) leading to a tidal pull that distorts their interiors and adds heat in addition to the heat generated internally by decay of radioactive elements contained within the rock.
Seeds of life?
With most or maybe even all of its seven known planets in the not-too-hot, not-too-cold “Goldilocks zone” around the star, TRAPPIST-1 offers the intriguing prospect of several Earth-like planets capable of hosting Earth-like life around the same star.
TRAPPIST-1 is young as ultracool dwarfs go, maybe only half a billion years old. But thanks to the frugal rate at which it uses its nuclear fuel it has a further 10 trillion years left to run (a thousand times longer then the sun). On Earth, it took two billion years to go from microbes to multi-cellular organisms, and another billion years for intelligence to emerge. So while we may not expect advanced civilisations to exist on the TRAPPIST-1 planets, some simple lifeforms may be in the works or already exist.
We don’t yet know how easy it is for life to get started even when conditions are right. But were life to exist or suddenly begin on any of TRAPPIST-1’s planets it is very likely that it would spread to its neighbours, as shown in a recent study.
Dwarf stars are more common than sun-like stars, and now we know they can have numerous Earth-like planets. It is beginning to look as if stars like our sun, a fast-burning “main sequence star”, may be less important as hosts for life-bearing planets than their stunted cousins.
TRAPPIST-1 and its planets are sure to be among the prime targets for the James Webb Space Telescope, likely to begin operations in 2019. This should be able to detect the presence of any atmosphere about a planet while it is in transit across the star and maybe even reveal whether atmospheric composition seems to have been modified by living processes.