What is the upper limit on how large a rocky planet can be? This is a contentious area in astronomy. Generally, it’s thought that above about 1.6 times the radius of Earth, a planet will become a gas giant. Anything below this is a super-Earth, and smaller still are the terrestrial planets the size of Earth, Mars, and so on.
But that boundary might need to be revised based on a new discovery. Astronomers say they have found a Neptune-sized exoplanet that is rocky in its composition. The planet, named BD+20594b, is an estimated 2.23 times the radius of Earth and 16.3 times its mass. If confirmed, this would be among the largest rocky planets we’ve ever found.
Found about 500 light-years from Earth, BD+20594b completes an orbit of its host Sun-like star every 42 days. “If confirmed to be rocky, the composition of BD+20594b would be unprecedented for exoplanets of this size,” the authors note in their paper on Arxiv. The research was led by Nestor Espinoza from the Pontifical Catholic University of Chile in Santiago.
The authors came to the conclusion that the planet is likely rocky because of its mass-to-radius ratio, from which its density can be calculated: 8 grams per cubic centimeter, compared to 5.5 for Earth. "We know it's rocky and not gaseous by its density," Espinoza told IFLScience. Namely, if the planet was gaseous and not rocky, it would have a larger radius given its mass.
Some caution is needed, though; the researchers noted that further studies will be needed to confirm its rocky composition, especially as it is at the hotly debated boundary between terrestrial planets and gas giants. "The errors on the mass are large!" added Espinoza. "However, I'm certain it is not gaseous. It might be an 'ocean' planet though, although the data is most consistent with a rocky planet."
The planet was discovered using the Kepler space telescope, as part of its current K2 mission, with follow-up observations from the HARPS spectrograph at the La Silla Observatory in Chile. And its discovery could have significant implications for our rapidly improving knowledge of exoplanets. Other suspected large rocky planets, or “mega Earths,” include Kepler-10c, 17.2 times the mass of Earth and 2.35 times its radius. However, its status as a rocky planet has been questioned. It seems the researchers are more confident that BD+20594b is one of the largest rocky planets we know of to date.
"BD+20594b is currently the only planet which could actually have the title of 'Mega-Earth,' a title we'll see if it actually has when we have more data at hand," Espinoza added.
[H/T: Universe Today]