Discovery of the most Earth-like exoplanet

Over the past 25 years, more than 4,000 exoplanets have been discovered, and sometimes surprises remain hidden in old data. By re-analyzing observations from the Kepler space telescope, researchers were able to confirm the existence of a planet that automatic analysis systems had classified as false positive. This planet is all the more interesting that it is of terrestrial size and in the habitable zone of its star. Be careful though, it is still far too early to say that we are dealing with a real Earth twin.

The system where this new planet is located is Kepler-1649, at 300 light years of the Earth in the constellation of the Swan. Thestar central is a red dwarf : her mass is 20% of that of Sun, its radius 23% of it and its temperature of 3,000 ° C, against 5,500 ° C for the Sun. With a magnitude about 16, it is ten thousand times too weak to be visible to theeye bare.

Kepler-1649, an already known system

This system is not unknown because a first planet, Kepler-1649 b, was found there a few years ago. Described as comparable to Venus by its size (1.02 ± 0.05 times the radius of the Earth, against 0.95 for Venus) and the light flux received from its star (2.2 ± 0.1 times that of the Earth, against 1, 9 for Venus), she traverses her orbit in 8.7 days and was spotted by Kepler thanks to his transits.

A planet of terrestrial size in habitable zone

Data from telescope spatial of the Nasa, whose mission ended in 2018, first analyzed by a pipeline dedicated, revealed for this system: Kepler-1649 b and another signal. However, the latter had been classified as a false positive, i.e. a signal produced by something other than a planet (for example, the variation of brightness intrinsic to the star). As part of the systematic visual inspection of these supposed false positives, a team of scientists led by Andrew Vanderburg re-analyzed the data from the Kepler-1649 system, which confirmed that this candidate was indeed due to a planet. Their results are described in a recent article appeared in The Astrophysical Journal Letters (open access sure arXiv).

This new planet, Kepler-1649 c, of size comparable to Earth – 1.06 (+ 0.15 / -0.10) in terrestrial radius, or 13,500 (+ 2,000 / -1,300) kilometers -, is located at 12 million kilometers from its star and travels its orbit in 19.5 days. His star host being small and cold, Kepler-1649 c receives approximately 75% of the luminous flux received from the Sun by the Earth, placing it in the habitable zone of its system, where a rocky planet could have water liquid.

Others exoplanets are probably closer to Earth in size, like Trappist-1 f and maybe Teegarden c, or in temperature, like Trappist-1 d and YOU 700 d. However, Kepler-1649 is the known planet most similar to Earth if we take these two aspects into account at the same time.

A cousin of the Earth? Not sure…

As the authors of the study point out, caution is advised as to the similarity between Kepler-1649 c and the Earth. Many characteristics of this planet remain little or not known, including its atmosphere, which can significantly affect its temperature. Furthermore, its environment is very different from that of Earth: in particular, red dwarfs are known to be eruptive (although no Kepler-1649 eruption has yet been detected) and Kepler-1649 is probably in synchronous rotation. Even if the planet probably has a rock composition (to be confirmed by measuring its composition), it could be very different from ours, and even if it will certainly be scrutinized for its astrobiological potential, it is far too early to say that Kepler-1649 it is a twin of the Earth.

A third planet?

The orbits of Kepler-1649 b and c have a special feature: while the first planet circles its star nine times, the second makes almost exactly four. Such reports of orbital period often correspond to resonances orbital, but a 9: 4 ratio is relatively unique among planetary systems. The resonances more often correspond to 2: 1 or 3: 2 ratios. This could indicate that an intermediate planet exists, with which the known planets are both in 3: 2 resonance. The absence of detection of this planet could be due to its small size or to an orbital inclination which means that it does not transit in front of its star.

This will also interest you

What do the exoplanets of Trappist-1 look like? This animation from NASA shows the possible portraits of the seven exoplanets discovered in February 2017 around Trappist-1, just 40 light years from our Solar System. These are interpretations because no one has seen them. They are assumed to be rocky and of similar size to that of Earth and no doubt differ from each other. Three are in the habitable zone and could therefore carry liquid water on the surface.

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