A team of astronomers from the Max Planck Institute for Solar System Research, the Georg August University of Göttingen and Sonneberg Observatory has developed a new sensitive algorithm to search for transiting planets in data from NASA’s Kepler Space Telescope. In the first test of their new algorithm, the researchers reanalyzed the archival data from the extended Kepler mission, K2, for 490 planet-hosting stars and discovered 18 new sub- to super-Earth-sized planets.
The 18 newly-discovered exoplanets (orange and green) are much smaller than Neptune, three of them even smaller than Earth and two more as large as Earth. Planet EPIC 201238110.02 is the only one of the new planets cool enough to potentially host liquid water on its surface. Image credit: NASA / JPL / NOAA / GSFC / Suomi NPP / 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 look for stars with periodically recurring drops in brightness. If a star happens to have a planet whose orbital plane is aligned with the line of sight from Earth, the planet occults a small fraction of the stellar light as it passes in front of the star once per orbit.
“Standard search algorithms attempt to identify sudden drops in brightness,” said Dr. René Heller, an astronomer at the Max Planck Institute for Solar System Research.
“In reality, however, a stellar 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 starlight than at the mid-time of the transit. The maximum dimming of the star occurs in the center of the transit just before the star becomes gradually brighter again.”
Large exoplanets tend to produce deep and clear brightness variations of their host stars so that the subtle center-to-limb brightness variation on the star hardly plays a role in their discovery.
Small exoplanets, however, present astronomers with immense challenges. Their effect on the stellar brightness is so small that it is extremely hard to distinguish from the natural brightness fluctuations of the star and from the noise that necessarily comes with any kind of observation.
“Our new algorithm helps to draw a more realistic picture of the exoplanet population in space,” said Dr. Michael Hippke, an astronomer with Sonneberg Observatory.
“This method constitutes a significant step forward, especially in the search for Earth-like planets.”
The 18 exoplanets discovered with the new algorithm are between 0.7 and 2.2 Earth radii, and half of them are smaller than about 1.2 Earth radii.
One of the new objects, EPIC 201497682.03, with a radius of 0.692 Earth radii is the second smallest planet ever discovered by the Kepler/K2 mission.
Another one, named K2-32e (EPIC 205071984e), has a radius only 1% larger than Earth’s.
“In most of the planetary systems that we studied, the new planets are the smallest,” said Dr. Kai Rodenbeck, an astronomer at the University of Göttingen and the Max Planck Institute for Solar System Research.
“What is more, most of the new planets orbit their star closer than their previously known planetary companions. The surfaces of these new planets therefore likely have temperatures well in excess of 100 degrees Celsius; some even have temperatures of up to 1,000 degrees Celsius.”
“Only one of the bodies, EPIC 201238110.02, is an exception. It likely orbits its red dwarf star within the so-called habitable zone. At this favorable distance from its star, this planet may offer conditions under which liquid water could occur on its surface – one of the basic prerequisites for life as we know it on Earth.”
The results are described in two papers released this month in the journal Astronomy & Astrophysics.
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René Heller et al. 2019. Transit least-squares survey I. Discovery and validation of an Earth-sized planet in the four-planet system K2-32 near the 1:2:5:7 resonance. A&A 625: A31; doi: 10.1051/0004-6361/201935276
René Heller et al. 2019. Transit least-squares survey. II. Discovery and validation of 17 new sub- to super-Earth-sized planets in multi-planet systems from K2. A&A, in press; doi: 10.1051/0004-6361/201935600
