"Our solar system is not as unique as we might have thought," speaks lead author Courtney Dressing of the Harvard-Smithsonian Center for Astrophysics (CfA). "It looks like stony exoplanets use the same basic constituents. To discover a truly Earth-like world, we should emphasis on planets less than 1.6 times the size of Earth, as those are the rocky worlds."
How do you create an Earth-like planet? The "test kitchen" of Earth has given us a thorough formula, but it wasn't clear if other planetary systems would follow the same recipe. Now, astrophysicists have found proof that the recipe for Earth also relates to terrestrial exoplanets circling distant stars.
Dressing presented the study today in a press release at a seminar of the American Astronomical Society.
The key to the finding was the HARPS-North instrument on the 3.6-meter Telescopio Nazionale Galileo in the Canary Islands. (HARPS stand for High-Accuracy Radial velocity Planet Searcher.) It is intended to precisely measure the masses of small, Earth-sized worlds. Those measurements are vital to find densities and therefore compositions.
"Our plan for using HARPS-North over the past year has been to emphasis on planets less than two times the diameter of Earth and to survey a few planets really well," clarifies Harvard astrophysicist David Charbonneau (CfA), who presently heads up the HARPS-North Science Team.
Most newly the team targeted Kepler-93b, a planet 1.5 times the size of Earth in a close-fitting, 4.7-day circle around its star. The mass and composition of this world were ambiguous. HARPS-North secured the mass at 4.02 times Earth, meaning that the planet has a rocky structure.
The scientists then associated all ten known exoplanets with a diameter less than 2.7 times Earth's that had precisely calculated masses. They discovered that the five planets with diameters smaller than 1.6 times Earth displayed a tight connection among mass and size. Moreover, Venus and Earth fit onto the same line, signifying that all these worlds have similar rock-iron compositions.
As for the bigger and more enormous exoplanets their densities showed to be considerably lower, showing that they comprise a large fraction of water or other volatiles, hydrogen and/or helium. They also displayed more diverse compositions rather than fitting into a single group like the smaller terrestrial worlds.
The team also distinguished that not all planets less than six times the mass of Earth are rocky. Some low-mass worlds with very low densities are also there (such as the planets in the Kepler-11 system). But for typical close-in small planets, the odds are high that they share an Earth-like structure.
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