The newly discovered star TRAPPIST-1 in the constellation Aquarius turns out to host seven Earth-like planets at once, with three of them at the centre of the 'life zone' and thought to possess water and a dense atmosphere, according to an article published in the journal Nature.
"Before the discovery of the TRAPPIST-1 system, we knew of only four real earth-like planets that we could study - Earth, Mars, Venus and Mercury. Now the number has increased by seven, and we can study them not five to ten years from now, but right now. We can expect to see the first news about their properties in the coming weeks and months. The TRAPPIST-1 story is just beginning," comments Michel Gillon, an astronomer at the University of Liège (Belgium).
The seven sisters
Last May, astronomers from MIT announced the discovery of a very unusual star system - TRAPPIST-1, just 40 light-years from Earth in the direction of the constellation Aquarius. At the time, three planets orbiting this red dwarf were reported to be inside the so-called "life zone", where water could exist in liquid form and their mass was thought to be comparable to Earth's.
Scientists later studied the spectrum of the TRAPPIST-1 star's rays as they passed through the planets' air shells on their way to Earth and found that they were rocky, Earth-like planets and also found hints of water, oxygen and carbon dioxide in their atmospheres. However, not all astronomers were convinced of the existence of these planets, noting that the periodic drops in the brightness of the star by which they were detected could have been generated by the invisible TRAPPIST-1 companion luminary.
Gillon and his colleagues studied the properties of these planets by observing the star system with the TRAPPIST telescope in Chile and the Spitzer orbiting telescope. As the scientists note, they anticipated that they could easily obtain information on the size, mass and composition of the planets' atmospheres due to the small distance to TRAPPIST-1, the quiet nature of the star and its small size, facilitating observations of the planets' shadows on its surface.
These observations have led to the unexpected discovery that there are in fact not three but seven planets, six of which are within the 'life zone'. For the first time, thanks to the high resolution of telescopes and long observations, planetary scientists have been able to measure both the diameter and mass of six of the seven planets very precisely and obtain some data on the composition of their atmospheres.
The riddle of the fifth planet
All the planets are similar to Earth in size - their radius is between 0.7 and 1.08 times the radius of our planet, and their mass is between 0.41 and 1.38. Accordingly, their density is equal to or slightly lower than Earth's, suggesting that they are rocky in nature or that they are oceanic planets.
Unlike our planet, the Seven Sisters orbit TRAPPIST-1 in a very tight orbit, with a year lasting from a day and a half to about two weeks. Even the last planet of the system, TRAPPIST-1h, is about four times closer to the star than Mercury is to the Sun.
That said, almost all the planets should be dominated by Earth-like or Mars-like weather, with average surface temperatures around zero or 20-30 degrees Fahrenheit. Only the first two planets break out of this trend, with temperatures in excess of 70-100 degrees Celsius, which probably makes them more similar to Venus than to Earth.
When it comes to the likelihood of life development, the three central planets - d, e and f - are the most likely to claim this role so far. According to Amaury Trio (Amaury Triaud) from the Astronomical Institute in Cambridge (UK), the best chance for the origin of life is the planet f, whose climate is soft and cool enough for it to exist water and organic matter.
It may not be long before the exact answer to this question - according to the trio, some features of the climate of planet f and its neighbours will become known in the coming years, and the first reliable data on its habitability can be obtained "within the current decade".
Are we alone in the Universe?
Two unusual properties of the TRAPPIST-1 system may influence the possibility of life on these planets, Gillon admits. He says all the planets are very likely to be captured by the tidal forces of the star, with the result that they always face one side of it. This can adversely affect the climate of the planets and make them virtually uninhabitable.
In addition, the planets themselves are in what is known as orbital resonance. Typically, this phenomenon results from the gravitational interaction of celestial bodies, which leads to the stabilisation of their orbits and the appearance of a strict mathematical relationship between their rotation periods. For example, Saturn and Jupiter are synchronized so that in two Saturnian years Jupiter makes exactly five rotations around the Sun.
