Find a planet with a dozen years of orbit in a few months

Design of the artist Kepler Space Telescope. Credit: NASA

To discover and confirm the presence of a planet around stars other than the Sun, astronomers expect to have achieved three orbits. However, this very effective technique has its drawbacks because it does not confirm the presence of planets at relatively long periods (it is ideally suited for periods of a few days to a few months).

To overcome this obstacle, a team of astronomers under the direction of the University of Geneva (UNIGE) have developed a method that ensures the presence of a planet in a few month, even if it takes 10 years to encircle his star: this new method is described for the first time in the newspaper Astronomy & Astrophysics .

The method of transits, consisting of detecting a drop in the luminosity of the host star at the moment of the passage of the planet, is a very effective technique for searching for exoplanets. It allows to estimate the radius of the planet, the inclination of the orbit and can be applied to a large number of stars at the same time.

However, it has a significant limitation: since it is necessary to wait at least three passages in front of the star to confirm the existence of a planet, it is currently suitable only for detecting planets with rather short orbital periods (typically from a few days to a few months). We will have to wait more than 30 years to detect a planet similar to Jupiter that needs 11 years to go around.

To overcome this obstacle, an astronomical team led by researcher Helen Giles, of the Department of Astronomy at the UNIGE Faculty of Science and member of the NCCR PlanetS, developed a method original. In analyzing data from the K2 space telescope, a star showed a significant temporary decay in brightness, the signature of a possible transit, ie the passage of a planet in front of its star. "We had to analyze hundreds of light curves," says the astronomer, to find one where such transit was unequivocal.

Helen Giles consulted recent data from the Gaia mission to determine the diameter of the star referenced EPIC248847494 and its distance, 1500 light years from the planet Earth. With this knowledge and the fact that the transit lasted 53 hours, she found that the planet is 4.5 times the distance from the Sun to the Earth, and so it takes about 10 years to orbit once .

Left to answer was if it was a planet and not a star. The Euler telescope of UNIGE in Chile would provide the answer. By measuring the radial velocity of the star, which allows to deduce the mass of the planet, she was able to show that the mass of the object is less than 13 times that of Jupiter – well below the minimum mass of a star ("This technique could be used to chase habitable planets resembling the Earth around stars like the Sun", enthuses Helen Giles, "we have already found Earth around red dwarf stars whose stellar radiation may have consequences on life that are not exactly known. "

With his method it will no longer be necessary to wait several years to find out if the only detected transit is due to the presence of a planet. "In the future, we could even see if the planet has one or more moons, like our Jupiter," she concludes.