This post is based on a fresh new article by “Bad Astronomer” Phil Plait in Slate magazine.
A team of astronomers has reported a discovery of the smallest exoplanet orbiting around a Sun-like star yet. The exoplanet Kepler-37b is a mere 3865 km (2400 miles) in diameter. That’s smaller than the planet Mercury is. That’s barely bigger than our own Moon, which is 3474 km across.
It was found in data taken by Kepler, which is searching among cca. 150,000 stars a day. “If a planet orbits the star, and the orbit of the planet aligns just right, from our viewpoint here at Earth we see the planet pass directly in front of the star. This is called a transit, and when it happens the star’s light dims a little bit. Most of the exoplanets we know of have been found this way.”
There are many ways how the signal can get confused. “The light from the star is compared to other stars, and if they change their light (by having planets themselves, or being intrinsically variable) that can look like a planet around the target star. The astronomers accounted for all that, and have a high degree of confidence the tiny planet exists.”
The parent star, named Kepler-37, is about 200 light years from Earth and is sort of cooler and smaller than our Sun. In total, three distinct planet signals were found in the starlight. The other two planets in this system are 0.75 and two times the diameter of the Earth, but they’re so close to their parent star they are extremely hot.
Our tiny planet Kepler-37b is so close to its star it’s definitely scorched and it’s probably a piece of rock without any atmosphere. In fact, it is hard to say if it even counts as a planet. It’s bigger than Pluto (which is about 2400 km across), but smaller than Mercury.
“We hadn’t seen anything this small until this one, so it’s at least telling us that bodies smaller than Mercury can form, even closer in to their stars.”
This planet was found as it was seen in the data by Kepler, but the size of the planet was uncertain because the size of its parent star was not certain. “You get the planet size by how much light it blocks from the star, but you need the star’s physical size to get that number.” So they used a technique called asteroseismology – it is mapping changes in the star’s brightness when the star oscillates. Interesting fact is that NASA didn’t fund the astroseismology work. “The team was, in part, crowd funded. A non-profit group called White Dwarf started the Pale Blue Dot project to raise money to help researchers. They let people “adopt” a Kepler star for a $10 donation, and then used that money to do science. They raised tens of thousands of dollars from this effort, and funded the asteroseismology used to measure Kepler-37b. And look what happened: They found out it was a record-breaking planet! Kepler-37 was in fact one of the stars adopted, by a user named SERGE.”