The first discovery of a planet around an alien sun – an extrasolar planet or exoplanet – was in 1995, and it was big news. By now we know thousands of them, so it takes something special to get into the news. In February 2017 we learned that one group had hit the jackpot: a star with seven Earth-sized planets, three of them in the habitable zone.

TRAPPIST-1 – not just a cool star, but an ultra-cool one
Hollywood has nothing to match TRAPPIST-1 for cool stars. It's an ultra-cool red dwarf star about forty light years away in the constellation Aquarius. The TRAPPIST robotic telescope in Chile discovered it. “TRAPPIST” is the TRAnsiting Planets and PlanetesImals Small Telescope project, located at the University of Liège in Belgium.

Sunlike stars have had a lot of attention, but red dwarfs are now also considered promising targets for planet hunters. They're by far the most common type of star and their planets are easier to detect. They're also the coolest and therefore longest-lived. TRAPPIST-1 has a temperature of about 2280°C (4130 °F). That sounds hot, but the Sun's temperature is 5500°C (9940 °F). Although the Sun has a life of about ten billion years, red dwarf stars may live for up to a hundred times that.

The TRAPPIST-1 system
Here is a diagram showing the TRAPPIST-1 system and the Solar System. At first glance they seem to be quite similar, but look carefully. The diagram of the TRAPPIST system has been enlarged by 25 times, and even then it just reaches to the orbit of Mars. The Sun has eight planets whose sizes vary dramatically. The whole TRAPPIST system of Earth-sized planets would fit within the orbit of Mercury.

The Goldilocks zone
We have to keep in mind that we only know one planet that has life on it – the Earth. Therefore we have to theorize from just one example. That's not good science, but it's the best we can do for now. However we do know that life needs an energy source. On Earth that's usually sunlight, but there are ecosystems deep in the ocean where no light penetrates. The foundation for these systems is energy released from chemical reactions of hydrogen sulphide.

We also know that life on Earth needs water, and that water is an excellent medium for biochemical reactions. This leads to the idea of a habitable zone around a star, the region which is neither too hot nor too cold for liquid water to exist. It's also called the Goldilocks zone.

The location of the habitable zone varies from star to star, and it varies over time, since stars may become hotter or cooler. TRAPPIST-1 is cooler than the Sun, so it's habitable zone is much closer to it than the Sun's.

How to find an exoplanet
The TRAPPIST-1 planets were discovered using the transit method. You look for the dip in a star's light that occurs when a planet passes in front of it. From this data, astronomers can determine the size and the orbital period of the planet. The planets also interact gravitationally, so Newton's theory of gravitation is applied to estimate their masses. The size and mass then give an estimate of density. Knowing its density means you can get a rough idea of a planet's composition.

The planets of TRAPPIST-1
An exoplanet usually has the name of its star plus a lower case letter. The first one discovered gets the letter b. (The star itself is the a member of the system.) If more than one planet is discovered at the same time, the planet nearest to the star is b. The TRAPPIST-1 planets go from TRAPPIST-1 b to the outermost TRAPPIST-1 h.

It seems likely that water would simply evaporate on TRAPPIST-1 a,b,c. Those planets are too hot. Any water on TRAPPIST-1 h would probably freeze. This planet is too cold. But there's a good chance that TRAPPIST-1 e,f,g are just right. They're in the Goldilocks zone, and could have large bodies of surface water.

The innermost six planets appear to be rocky. The orbit of TRAPPIST-1 h is still unresolved and its mass is uncertain. But, interestingly, TRAPPIST-1 f seems to be noticeably less dense than the others. This implies that it has less rock and iron and probably more water. It could be a watery world.

It's surprising that we can find out so much about worlds so far away. And yet the speculation has been more substantial than the facts, which are still few. A number of telescopes will be studying the TRAPPIST-1 system to tease out more information and strengthen the data we have.

Life?
Are there simple life forms on any of these planets? No one knows, and getting evidence for it will be difficult. However this nice set of planets, similar yet different, should keep astronomers busy for many years studying them and matching the data with models.

Some people have seized on the possibility of life on one or more of these planets. (It makes good headlines.) Others are doubtful about this. A future article will look at some of the arguments for and against the likely habitability of the TRAPPIST-1 planets.

Dated 2017-03-24