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A team of astronomers from the Max Planck Institute for Solar System Research, the Georg August University of Göttingen and the Sonneberg Observatory has developed a new sensitive algorithm for searching planets in transit in the data of the NASA Kepler Space Telescope. In the first test of their new algorithm, researchers performed a new analysis of the Kepler extended mission archive data, K2, for 490 stars hosting planets, and discovered 18 new planets of the size of 39, a super-Earth.
The 18 newly discovered exoplanets (orange and green) are much smaller than Neptune, three of them even smaller than the Earth and two others as large as the Earth. Planet EPIC 201238110.02 is the only one of the new planets cold enough to possibly harbor liquid water on its surface. Image credit: NASA / JPL / NOAA / GSFC / Suomi Nuclear Power Plant / VIIRS / Norman Kuring / Max Planck Institute for Solar System Research / René Heller.
In their search for distant worlds, astronomers often use the so-called transit method to search for stars with recurrent luminosity drops. 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 attempt to identify sudden drops in luminosity," said Dr. René Heller, astronomer at the Max Planck Institute for Solar System Research.
"In reality, a star disk appears slightly darker at the edge than in the center. When a planet moves in front of a star, it therefore initially blocks less stellar light than in the middle of the transit. The maximum gradation of the star occurs at the center of the transit, just before the star becomes progressively brighter again. "
Large exoplanets tend to produce deep and sharp variations in luminosity from their host stars, so the subtle variation in center-to-branch brightness plays an important role in their discovery.
Small exoplanets, however, pose enormous challenges to astronomers. 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.
"Our new algorithm helps to draw a more realistic picture of the population of exoplanets in space," said Dr. Michael Hippke, astronomer at the Sonneberg Observatory.
"This method is a significant step forward, especially in the search for Earth-like planets."
The 18 exoplanets discovered with the new algorithm lie between 0.7 and 2.2 Earth's rays, and half of them are less than about 1.2 Earth's radius.
One of the new objects, EPIC 201497682.03, with a radius of 0.692 Earth radius is the second smallest planet ever discovered by the Kepler / K2 mission.
Another, named K2-32e (EPIC 205071984e), has a radius of only 1% greater than that of the Earth.
"In most of the planetary systems we have studied, the new planets are the smallest," said Dr. Kai Rodenbeck, astronomer at the University of Göttingen and at the Max Planck Institute for Research on solar systems.
"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 1,000 degrees Celsius. "
"Only one of the organizations, EPIC 201238110.02, is an exception. He is probably orbiting his 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 prerequisites for life as we know it on Earth. "
The results are described in two articles published this month in the journal Astronomy and astrophysics.
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René Heller et al. 2019. Least Squares Survey in Transit I. Discovery and validation of a planet of Earth's size in the four-planet K2-32 near resonance 1: 2: 5: 7 system. A & A 625: A31; do I: 10.1051 / 0004-6361 / 201935276
René Heller et al. 2019. Least Squares in Transit Survey. II. Discovery and validation of 17 new sub-to-super-terrestrial planets in multi-planet K2 systems. A & A, in the press; do I: 10.1051 / 0004-6361 / 201935600
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