Expolanets: 7 cousins ​​of the Earth discovered around the same star

This system is composed of 7 planets of sizes and masses comparable to those of the Earth, which revolve around a red dwarf, a not massive star whose low temperature makes possible the presence of liquid water on their surface.

This spectacular discovery, which is the subject of a publication in the journal Nature, raises great hopes.

Why is this discovery important? Video :

Unlike most astronomers who have so far pointed their telescopes at stars similar to the Sun, the team led by Michaël Gillon, Space Science, Technologies and Astrophysics Research (STAR) of the University of Liège, Belgium , rather directed his attention on not very massive stars, like the red dwarfs, which abound in our galaxy.

And so this international team detected and confirmed the existence of seven Earth-sized planets as they passed a star located 39 light-years from us in the constellation Aquarius. Called TRAPPIST-1, this star has a mass equivalent to about 8% of that of the Sun and a size slightly larger than that of the planet Jupiter. It emits therefore a much lower energy than that of the Sun.

Detected by the transit method

The researchers have thus detected the existence of these seven planets by the method of transits. Whenever a planet passed TRAPPIST-1, it blocked a small portion of the light emitted by the star in our direction. In addition, the fact that this slight obscuration occurred periodically confirmed that it was indeed a planet orbiting this star.

The observation of 92 planetary transits – the passage of a planet in front of its star – allowed astronomers to identify seven distinct planets around TRAPPIST-1, which they named TRAPPIST-1b, c, d, e, f, g and h, in order of increasing distance from their parent star.

When stars are much smaller than the Sun, planets the size of the Earth project a relatively larger shadow, which makes it easier to detect and study them,” said a press briefing. Amaury Triaud, researcher at the Institute of Astronomy at Cambridge University in the United Kingdom and co-author of the article

“By choosing to fix our telescopes on a small star, like TRAPPIST-1, we were able to get a magnification 80 times larger than we would have had with a star the size of the Sun. This means that we will be able to study in detail the climate of these planets, as well as the chemical composition of their atmosphere, which will indicate to us if there is life. We should know in ten years if there is indeed life on these planets.

Researchers made their observations from NASA’s Spitzer Space Telescope, as well as from multiple terrestrial telescopes, such as the TRAPPIST-South (TRAnsiting Planets and Planetsimals Small Telescopes) of the European Southern La Silla Observatory. Observatory (ESO), in Chile, the TRAPPIST-North of the Oukaïmeden Observatory, Morocco, the Very Large Telescope / HAWK-1 of the Paramal Observatory, in Chile, the UKIRT telescope at Mauna Kea, Hawaii, the telescope William Herschel in La Palma, Canary Islands, the Liverpool telescope and the SAAO telescope of the South African Astronomical Observatory (SAAO).

These observations allowed them to determine the size, mass, orbit and temperature of each of these planets, or even the density for six of them. It thus appeared that the size of five planets (b, c, e, f, g) is similar to that of the Earth, while that of the two others (d and h) is a little smaller, that is intermediate between those of Mars and that of the Earth. Estimates of the mass of the six planets b, c, d, e, f, g suggest that they are likely rocky.

“This planetary system is very compact,” says Michaël Guillon. The planets are very close to their star and located very close to each other. “

The planet TRAPPIST-1b goes around its star in a day and a half, its neighbor does it in two and a half days, the third in four days, the fourth in six days, the fifth in nine days, the sixth in twelve days and the seventh in twenty days. “TRAPPIST-1 is a star so small and so cold that the seven planets are temperate, which means that liquid water could be on their surface and, therefore, possibly life,” he says. there.

This compact configuration of the system makes the heart of these planets likely to experience tidal effects under the influence of the gravitational force of their star which, because it is very close to them, draws them towards it, as well as only by the gravitational force of the neighboring planets which are also very close to each other. “Subject to these opposing forces, the interior of the planets is massaged, creating a strong friction that generates heat, which helps to maintain a surface temperature allowing the presence of liquid water”, explains Robert Lamontagne, astronomer to the University of Montreal.

The researchers also point out that the orbital periods – the time the planet takes to circle its star – of the seven planets are almost complete multiples of each other and that for this reason the system is ” in almost resonance. “Their rotation is thus synchronous with their period of revolution [around the star], which means that the planets always have the same face to their star, like our moon. So they have a side that is immersed in a permanent night and a side constantly lit. Nevertheless, climate models suggest that there is enough circulation to temper the climate of these planets, “says René Doyon, astrophysicist at the University of Montreal.

Liquid water (and life)?

Using a climate model incorporating the low temperature of the star, the team of researchers estimated that the planets e, f and g could present oceans of liquid water on their surface as well as a terrestrial atmosphere. . These three planets are indeed in the so-called habitable zone, where the conditions are particularly favorable to the appearance of life.

Another model assuming that the seven planets exhibit a certain synchronicity in their orbital periods predicts that the three planets (b, c, and d) closest to the star would also contain liquid water in a larger quantity. limited in certain areas of their surface.

We should find many other planet sisters of the Earth in the coming years

Robert Lamontagne, astronomer at the University of Montreal

“It is not the first exoplanets of size and mass comparable to the Earth that we discover, but two of the seven planets of the TRAPPIST-1 system are at the right distance from their star to have a temperature comparable to that prevailing on Earth, which means that if there is water on these planets, it would probably be liquid, “says Robert Lamontagne, astronomer at the University of Montreal. “As these two planets go around their star in a few days, they pass in front of their star every four days for one and every six days for the other, so that we can observe them regularly and very easily. The other interesting aspect is the fact that this system is very close to us, less than 40 light-years away, which facilitates its observation.

“As these planets will be easier to study, it will be easier to detect a chemical signature typical of the presence of life in the composition of their atmosphere,” he adds.

Expolanets: 7 cousins ​​of the Earth discovered around the same star

Photo: NASA / JPL-CaltechAn illustration of the TRAPPIST-1 system and the possible state of water on the surface of planets that orbit the star

According to Michaël Gillon, “if we find a combination of methane, oxygen, ozone and carbon dioxide, we will have a good indication of the presence of a biological activity”.

A great moment in the study of exoplanets

For René Doyon, this discovery is “exciting” because of the proximity of the star TRAPPIST-1. “It’s one thing to find terrestrial planets, but you also have to be able to study them,” he says. Moreover, the planets of this system transit between the Earth and their star, which makes it possible to study their atmosphere. To do this, we first analyze the spectrum of the star and then, when the planet passes in front of the star, its atmosphere filters the light of the star and can thus detect the footprint of the atmosphere. This is how one determines whether they have an atmosphere and the composition of it. Also, the fact that their star is very small makes it easier to study their atmosphere. “

But what animates above all the astrophysicist is the fact that the imager in the near infrared and spectrograph without slot, called NIRISS (Near-Infrared Imager and Slitless Spectrograph), that his team of the University of Montreal has put developed to be installed on the future James Webb space telescope, will study the atmosphere of two or three of these new exoplanets located in the habitable zone, as early as the summer of 2019.

“We understand that this TRAPPIST-1 system is very different from the solar system,” note Mr. Lamontagne and Mr. Doyon.

The TRAPPIST-1 system is more like Jupiter’s moons system. But “those stars, like TRAPPIST-1, which are smaller and colder than the Sun are the norm in the galaxy,” says Lamontagne. Which means that we should find many other planet sisters of the Earth in the coming years. “

Review our discussion with astrophysicist Robert LamontagneWhat’s this discovery? We spoke with the coordinator of the Center for Research in Astrophysics of Quebec.

See a 360 ° panorama of what the surface of TRAPPIST-1d might look like.

Expolanets: 7 cousins ​​of the Earth discovered around the same star

Artist’s impression of the planet Trappist 1-f, located 39 light-years from Earth. – NASA / JPL-Caltech

Stars that burn almost forever

The problem is that by being so close, the planets are probably bombarded by radiation. But while the scientists thought that this was incompatible with the presence of liquid water due to an extreme greenhouse effect, the researchers led by Michaël Gillon believe that they can, in theory, be protected by their atmosphere.

Trappist-1 is indeed rather calm, and it has another potentially crucial feature: while the death of the Sun will begin in 5 billion years, this dwarf star, which burns its hydrogen slowly, will continue to exist for 10,000 billion years. At present, 39 light-years remains an insurmountable distance. But in the long run, Trappist-1, or one of the 50 stars within 15 light-years of Earth, may well be a life-saving “plan B” for men

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