A total of 18 Earth-sized planets have recently been found orbiting alien stars, hiding in data collected by the now-defunct Kepler spacecraft. One of these is the smallest exoplanet yet discovered, and another might have conditions favorable to life.
A large fraction of exoplanets are discovered by observing a drop in the brightness of a star as a planet orbits, passing between its stellar companion and the Earth. This transit method of detecting planets works best with larger planets, which block out more starlight than smaller worlds, and show distinct shadows. Stars appear to be brighter at their centers than near their edges, providing another challenge when trying to find smaller exoplanets. Researchers at the Max Planck Institute for Solar System Research (MPS) developed an algorithm with the ability to recognize the tell-tale signatures of small worlds.
“In most of the planetary systems that we studied, the new planets are the smallest,” Kai Rodenbeck of the University of Göttingen stated.
This newly-developed algorithm from Max Planck looks for smaller worlds which may have been missed in data from Kepler and other observatories searching for worlds beyond our Solar System. The first test sample of the new process looked at 517 stars in Kepler data, where at least one transiting planet has already been found.
The smallest of the worlds detected in this new study is just 69 percent as large as Earth, while the largest is merely twice the size of our own world. The vast majority of the planets discovered in this new study orbit close to their stars, subjecting these worlds to tremendous heat. Surface temperatures on these planets could be as high as 100 to 1000 degrees Celsius (roughly 200 to 1800 degrees Fahrenheit).
Roughly 96 percent of the 4,000 or so exoplanets discovered, so far, are significantly larger than the Earth. Not only do larger exoplanets block out more starlight than smaller worlds, they also create sharper shadows — something earlier search methods looked for to reveal the presence of a target. Smaller worlds show a less distinct change in brightness as transit takes place.
“…[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,” Dr. Rene Heller from MPS, describes.
From 2009 to 2013, the Kepler space telescope searched 100,000 stars, discovering 2,300 worlds in alien solar systems. Following an equipment failure, the orbiting observatory was rebranded K2, and a new, modified mission was born. By the time the spacecraft was shut down during the autumn of 2018, Kepler had survived for a long second life, studying more than 100,000 additional stars.
Only one world revealed in this new study may be habitable for life as we known it. EPIC 201238110.02 may not have the most welcoming name, but this could be the most hospitable of any of these worlds. Larger than the Earth, but smaller than Neptune, this planet is found within the habitable zone from its sun. Being neither too close, nor to far, from its stellar parent could permit liquid water to pool on its surface.
Before 1992, we did not know of a single world beyond our own Solar System. Kepler discovered thousands. Today, the Transiting Exoplanet Survey Satellite (TESS) is already hard at work adding to our catalog of known alien planets.
Researchers at MPS believe their new algorithm may eventually find as many as 100 unknown worlds hiding in data we have seen, but never before recognized — until now.