At half the size of Neptune, it should be made of gas — but it’s not!
This is one big ball of rock! Exoplanet BD+20594b, discovered in 2015, may be the biggest known rocky world discovered so far in the universe. It is roughly half the diameter of Neptune, but it is 100 percent rock according to researchers who posted their paper online at arXIV.org on January 28.
Ever since the 1992 discovery of the first rocky exoplanet — a term used describe planets with compositions ranging between 100 percent MgSiO3 (Magnesium metasilicate) and 100 percent Fe (Iron) — astronomers have been trying to not only find more of them, but also study their formation, composition and evolution, since they resemble Earth in several ways.
Although the number of rocky exoplanets discovered so far is small, there are some interesting relationships that suggest these planets have common properties. For example, these Earth-like planets have similar compositions, but contain small, naturally occurring differences.
However, enter BD+20594b, a potential exception to the previously discovered rocky worlds: a Neptune-sized exoplanet that is 100 percent pure rock. So why does this newly found planet seem to be defying the rules?
It turns out that BD+20594b, due to its large size, was expected to have a low density and be composed of large amounts of volatiles (gases). However, its large mass means it is made up of significant amounts of rock (MgSiO3), and only small amounts of Fe and volatiles. In fact, researchers estimate it to be a pure rock composition — placing it in a strange, limbo-like position between rocky and non-rocky planets.
BD+20594b is located about 500 light-years from the constellation Aries. It is about 16 times the size of Earth, however it is only a little over twice as wide — making its density about 8 grams per cubic centimeter, according to Néstor Espinoza, an astrophysicist at the Pontifical Catholic University of Chile in Santiago and lead author of the paper.
Earth’s density, by comparison, is 5.5 grams per cubic centimeter.
Kepler-10c, a rocky “mega Earth” planet spotted in 2014, is comparable to BD+20594b. Kepler is 2.4 times as wide as Earth with a mass equal to nearly 17 Earths. However, recent measurements have indicated that Kepler-10c isn’t as large as thought — only 14 times as massive as Earth — meaning the planet is probably encased in a shell of gas or water.
Interestingly, the large mass of BD+20594b implies that if the planet ever had the chance to acquire an atmosphere, it would keep it. But the question is, if it did have an atmosphere at one point, where did it go?
Given the planet’s distance from its parent star, its atmosphere likely wasn’t removed by solar radiation. It might be possible that the planet formed after the dissipation of the gas in the protoplanetary disk or that some external force removed the atmosphere — such as a giant impact.
It this planet is confirmed to be 100 percent rock, it would be unprecedented for exoplanets of this size.