Another day, another planet, right? Astronomers have discovered 861 new planets beyond the solar system. Most are the large, Jupiter-sized variety because they’re easiest to detect. A few are Earth-sized and considerably harder to see.
Get ready for a new milestone. In an article published in the Journal Nature, a team of astronomers describes their discovery of a planet only 2,400 miles in diameter. That’s hardly bigger than the moon (2,160 miles) and smaller than the planet Mercury.
Named Kepler-37b, it orbits a star in the constellation Lyra the Harp some 210 light years from Earth. The moon-sized planet and two larger companion planets were found by NASA’s Kepler space telescope. With an unblinking eye, Kepler stares at more than 150,000 stars making measurements of their brightness every 30 minutes.
When a potential planet orbits in front of its host star, it blocks a small percentage of the star’s light. Astronomers measure a dip in brightness which tells them the planet’s size relative to the star.
In the case of Kepler 37-b, to pin down the planet’s diameter, they accurately measured the size of the host star using a relatively new technique called asteroseismology.
The same way geologists use seismic waves to study the interior of the Earth, astronomers use sound waves generated by the boiling motions of hot gases in the star’s interior. Sound waves create “starquakes” that make the surface oscillate up and down and ring like a bell. Kepler observes the vibrations as a rapid flickering of the star’s light.
Using high-precision instruments, the scope determined the host star’s size to an accuracy of 3%. With that number in hand, calculating the planet’s diameter was a straightforward exercise.
While Kepler’s primary mission is to search for planets in a star’s “habitable zone” where liquid water is stable on a planet’s surface, it turns up all kinds of wild worlds. Kepler-37b orbits Kepler-37 every 13 days at a distance of only about 12 million miles or three times closer than Mercury is to the sun. It’s a hot, rocky world with a furnace-like surface temperature of over 800 degrees F. Not a place where life might easily likely a foothold.
Kepler-37’s solar system is quite a bit different from the one we know best. Its other two planets, Kepler 37-c and 37-d, also orbit closer in than Mercury. Despite being within our planet’s size range, being so close to their host sun, they’re continuously baked. No Earth-like planets here.
So you might naturally ask, how does this moon-sized object rate as a planet? What about Pluto? Well, it does orbit a star, and its size tells us it’s spherical in shape, but whether it “clears it orbit” of other smaller bodies, as the current definition of planet requires, we may never know. My gut feeling tells me we’re in a state of transition about what defines a planet. Sooner or later, astronomers will have to take a look at the big picture that now includes hundreds of new planets and 128 multiple-planet solar systems and decide on something more comprehensive. Perhaps little Kepler-37b will nudge us in that direction.