Scientists used a new method to find small exoplanets, neglected in previous surveys – ScienceDaily

The 18 worlds recently discovered fall into the category of planets the size of the Earth. The smallest of them represents only 69% of the size of the Earth; the largest is barely more than twice the radius of the Earth. And they have another thing in common: the 18 planets have not yet been detected in the Kepler Space Telescope data. Current search algorithms were not sensitive enough.

In their quest for distant worlds, scientists often use the so-called transit method to search for stars with recurring drops in brightness. If a star has a planet whose orbital plane is aligned with the Earth's line of sight, the planet obscures a small fraction of the starlight when it passes the star once in orbit.

"Standard search algorithms try to identify sudden drops in brightness," says Dr. Rene Heller of MPS, the first author of current publications. "In reality, however, a star disk appears slightly darker at the edge than in the center.When a planet moves in front of a star, it initially blocks less starlight than in the middle of the transit." The maximum gradation of the star appears at the center of the transit just before it gradually becomes more brilliant, "he explains.

Large planets tend to produce deep and sharp variations in brightness from their host stars, so the subtle variation in brightness from the center to the star's branches plays virtually no role in their discovery. Smaller planets, however, pose immense challenges to scientists. Their effect on stellar luminosity is so small that it is extremely difficult to distinguish between the natural fluctuations of the brightness of the star and the noise that necessarily accompanies any type of observation. The team of Rene Heller has now been able to show that the sensitivity of the transit method can be significantly improved if a more realistic light curve is assumed in the search algorithm.

"Our new algorithm helps to draw a more realistic picture of the population of exoplanets in space," sums up Michael Hippke of the Sonneberg Observatory. "This method is an important step forward, especially in the search for planets similar to the Earth."

The researchers used NASA's Kepler Space Telescope data as a test bench for their new algorithm. During the first mission phase from 2009 to 2013, Kepler recorded the light curves of more than 100,000 stars, revealing more than 2,300 planets. After a technical defect, the telescope had to be used in another mode of observation, called mission K2, but it nevertheless monitored more than 100,000 stars at the end of the mission in 2018. First test sample of their New algorithm, The researchers decided to re-analyze all 517 K2 stars that already hosted at least one planet in transit.

In addition to the previously known planets, the researchers discovered 18 new objects that had been previously neglected. "In most of the planetary systems we have studied, the new planets are the smallest," says co-author Kai Rodenbeck of the University of Göttingen, who describes the results. What's more, most new planets gravitate around their star closer to their previously known planetary companions. The surfaces of these new planets therefore probably have temperatures well above 100 degrees Celsius; some even have temperatures of up to 1000 degrees Celsius. A single body is an exception: it probably orbits its red dwarf star in the so-called habitable zone. At this favorable distance from its star, this planet can offer conditions in which liquid water can form on its surface – one of the basic conditions of life as we know it on Earth.

Of course, researchers can not exclude that their method is also blind to other planets in the systems they have studied. In particular, small planets far from their host stars are problematic. They need more time to perform a complete orbit than the planets that orbit their stars more closely. As a result, transits of planets into wide orbits occur less often, making their signals even more difficult to detect.

The new method developed by Heller and his colleagues opens fascinating possibilities. In addition to the 517 stars in the study, the Kepler mission also offers datasets for hundreds of thousands of other stars. The researchers assume that their method will allow them to find more than 100 other worlds of the size of the Earth in Kepler's main mission data. "This new method is also particularly useful for preparing the next PLATO mission to be launched in 2026 by the European Space Agency," says Professor Laurent Gizon, MPS Director General. PLATO will discover and characterize many other multi-planet systems around Sun-like stars, some of which will be able to house life.