Vol. XLI No. 10 March 05, 2017

Earth's Seven Sisters 40 Light-years Away

Prabir Purkayastha

AN international team of scientists have discovered a set of seven, near earth-sized exoplanets circling a nearby star, named Trappist-1. They have reported this discovery recently in Nature. What is exciting about these exoplanets is that all of them may have water, with three falling in what is called the habitable zone. In the habitable zone, planets may have water in liquid form on their surface, and therefore the possibility of life. While this is not the same as finding life in the galaxy, discovering near earth-like planets with the possibility of water only 40 light-years away, does bring us closer to the discovery of extraterrestrial life.

First, what are exoplanets? Exoplanets are planets that are outside our familiar solar system. They circle other stars. These seven exoplanets belong to a star that is only 40 light-years from earth. We can therefore study them much more closely than others that are much further away. This, combined with their having the possibility of water and therefore life, has raised the excitement in the scientific community.

The story of the star Trappist-1 is interesting on other counts as well. It was initially discovered by a team of Belgian scientists from Liege University, who used a relatively small and inexpensive telescope, only about 60 centimetres (or 2 feet) in diameter named Trappist. The star is named after the telescope. You can mount such a telescope in your garage! Compared to costs of giant telescopes such as the Hubble telescope, it is only a small fraction.  Good science can still be done with much less money!

Trappist was installed in 2010, at La Silla, the European Southern Observatory, located at the edge of Atacama desert in Chile, and one of the best places for viewing the night sky. This small telescope is fully automated, records the night sky on a camera, and is operated remotely from a control room in Liege, Belgium 12,000 kilometres away. The name Trappist (Transiting Planets and Planetesimals Small Telescope) refers to a popular Belgian beer, and to remind the people that the telescope is Belgian.

The lead author of the Nature paper, is Michael Gilon, a professor in University of Liege, Belgium, who postulated that instead of searching for life only on exoplanets around sun-like stars, we could also search for exoplanets circling ultra-cool, red dwarf stars. Such stars are much more in number, and if they have earth-like planets, we are likely to find them much closer to us, making their study that much easier. There are 500 such red dwarfs within only 100 light-years of us, and therefore giving us a number of targets for such study. It also makes it possible to use much cheaper telescopes.

Gilon's intuition was startlingly fulfilled, when Trappist team observed what they thought were three earth-sized planets in a habitable zone around a star only 40 light-years away. This is a very short distance by astronomical standards. They reported this discovery in May 2016, immediately raising interest of the global astronomical community. Many others joined the hunt for habitable exoplanets around Trappist-1, including NASA with its Spitzer telescope.  The joint exercise carried out by a number of teams and telescopes, determined that there were actually seven, and not three plants as originally thought that were circling Trappist-1. And all of them have the possibility of water, with three falling within the habitable zone.

The excitement is not just about finding habitable planets. It shows that we now have a much larger number of stars that may have earth-like planets: there are three times the number of red dwarfs as there are yellow stars like our sun. Secondly, since Trappist-1 is so close, we can study it more easily, using other more sophisticated telescopes that are being now launched. These telescopes can not only detect the presence of water vapour in the atmosphere, but also the presence of oxygen, methane and ozone – or bio-signatures – in the atmosphere of these planets. While this will not prove the existence of life on the Trappist planets, they will make this possibility greater.

While the red-dwarfs are more frequent than the sun-like stars, they are also weirder. They are considerably smaller. Trappist-1 is only 8 percent of the size of the sun and a little larger than Jupiter. This also means that the planets are much closer to their mother stars, and therefore have much smaller orbits and orbital periods. All the seven planets are bunched very close together, and complete their orbits – their year – within 1.5 to 20 days.

The smaller the orbit of these exoplanets, they come between us and the star – our line of sight to the star – much more frequently. This is what the astronomers call transit. A smaller orbit, means a much higher number of transits in a given period, and therefore many more observations. As they travel between us and their star, they block the light from the star, and can be seen as a black dot moving across the star. The size of the dot and other measurements can generate a lot of information, apart from the simple one of determining their orbiting time. This how we now know the size, mass, time for orbiting of these seven exoplanets.

So the next important question, what are the likelihoods of life on Trappist-1 planets? At the moment, the answers are entirely speculative. The simple answer is we know very little about the conditions on the Trappist-1 planets; or the conditions near a red dwarf star such as Trappist-1.

 Trappist-1 is a young star -- only 500 billion years old – unlike the sun, which is 4.5 billion years, or nine times its age. NASA says, “Such stars are extremely active in their early years—shooting off potentially lethal flares and bursts of radiation” (Earth-sized planets: The newest, weirdest generation). Such radiation could potentially kill all life forms; or strip away all the oxygen from the planets; or lead to runway greenhouse effect, leading to very high temperatures.

The optimists argue that we only know about life on earth. There may be completely different forms of life that can survive high radiation or other dangers that would kill us.

Without detailed knowledge, the scientific community is currently swinging from one extreme to the other. We just do not know enough to hazard a guess. Take the case of our solar system. From a distance, Venus, Earth and Mars are planets that all lie within the habitable zone, but have very different atmospheres. Unless we have more data from future studies, we can only imagine the possibilities.

While 40 light-years is a short distance in astronomical terms, it is still far beyond the possibility of an inter-stellar mission. Using known technology, it would take us anything from 160,000 to 700,000 years to reach Trappist-1. Just to put it in perspective, 160,000 years is roughly the span of homo sapiens – our species – on earth. So Star Trek alas, is still for TV; or the movie halls.

Despite the problems for life on Trappist-1, the interest is that these are by far the most interesting planets we have discovered till date. And they are at a distance that allows us to study them closely in the coming years.

Let us remember that not only does the earth we know have an atmosphere, but we are altering this atmosphere in measurable ways that can be observed from other stars. If there is life on any of the Trappist planets, we can study its atmosphere for similar signs. This is our best hope of discovering life outside the solar system. And Trappist-1 lies at a distance that makes such studies possible. That is why the excitement over its seven planets.