It’s Earth-sized, It’s Rocky…

But, it’s Too Hot to Support Life

It had to happen eventually—the discovery of an exoplanet with the same rocky composition and size as Earth, but orbiting another star. The planet and its star were first observed by the Kepler space telescope. Astronomers targeted this world, called Kepler-78b, for followup observations using the W.M. Keck Observatory on Mauna Kea, on the Big Island of Hawai’i. It lies about 400 light-years from Earth in the direction of the constellation Cygnus, the Swan.

The planet has some peculiar characteristics, as does its star, which rotates very slowly—about 1.5 meters per second. At that speed the star completes one rotation on its axis every 12.5 days. To give you an idea of how slow that is, a typical jogger goes about the same speed here on Earth. In addition, the parent star for Kepler-78b has star spots, which make it more difficult to suss out any possible planets. Astronomers had to do quite a bit of observational analysis to separate out the planet in the observations.

Kepler-78b has a radius of 1.2 times Earth’s, and has about 1.7 times Earth’s mass. It follows an 8.5-hour orbit which is incredibly fast compared to Earth’s 365-day trip around the Sun. For the planet to orbit that fast, it lies about 15,000,000 kilometers from its star.That’s amazingly close, which explains why this planet is too hot to support life. By comparison, Mercury lies more than 59 million kilometers from the Sun.

As astronomers find more and more planets in the Kepler data, they’re able to classify them in different categories, such as “superEarth” and “hot Jupiter”. Kepler-78b is in a relatively new class called “ultrashort period planets. These are worlds discovered in Kepler observations that have orbital periods of less than 12 hours. They’re small, usually about one or two times the size of Earth, and all are so close to their planets that their surfaces are being scorched. It’s not clear how these rocky little worlds formed and survived in such close proximity to their stars.

This finding is just one of many discoveries flowing from the Kepler mission. Mechanical issues have ended the spacecraft’s most sensitive planet searches, but there is still plenty of work to do analyzing its vast treasure trove of information. Learn more about this most recent finding here.