The sci-fi worlds of Star Wars and Star Trek have trained us to think there are rocky, solid planets orbiting far away stars that are so similar to Earth we could walk upright and breathe their atmosphere. In reality, scientists have yet to identify a single planet outside of our solar system that's comparable to the size and composition of Earth.
Until now, that is.
Two independent research groups, reporting in Nature, have found that a far-off planet, Kepler-78b, has a similar mass and composition to Earth. While previous observations of Kepler-78b had estimated the size of the planet to be 1.2 times wider than Earth, these latest measurements focused on observing the gravitational interactions between the planet and its star. The result? Researchers can combine the previously known size of the planet with the newly measured mass to determine its makeup.
The gravitational interactions between the planet and its star
Of course a star has a strong gravitational pull on a planet — that's why planets orbit stars. But planets also exert a gravitational force on their host star, causing a small wobble in the star's position. This wobble can be seen as a change in the wavelength of the light coming from the star, which is analogous to the change in sound of a train passing you — the pitch drops (the wavelength changes) because of the motion of the train. Researchers used this wobble to conclude that Kepler-78b has a mass 1.69 times larger than that of Earth. The combination of the size and mass of the planet yields the planet's density, allowing researchers to determine what combination of materials comprise the planet. The density of Kepler-78b, they found, corresponds to an Earth-like makeup of rock and iron.
An Earthlike makeup of rock and iron
"Having a planet made largely of rock and iron, like our Earth, means that we have a good surface for life to evolve on," says Andrew Howard, one of the researchers behind the new findings. Unfortunately, the composition of Kepler-78b doesn't necessarily make it hospitable to life. "From an Earth-centric perspective, we'd like to have liquid water on the surface [and] this planet is obviously way too hot."
Indeed, Kepler-78b is about 100 times closer to its star than we are to the sun, and it orbits the star in a blazingly fast 8.5 hours. As a consequence, the planet's surface temperature is somewhere between 3700 and 5100ºF. That heat, combined with huge amounts of UV radiation, means the planet could not support an Earthlike atmosphere, nor any liquid surface water. "It's probably a lava planet, so the surface is just red lava flowing. It's completely different [from Earth]," said Francesco Pepe, lead author of the second study. "The sun would occupy half of the sky."
"The surface is just red lava flowing."
Kepler-78b might not be habitable, but scientists aren't done studying the mysterious planet. "We have now more questions we have to answer," Pepe says, noting that the particular geometry of this planet–star pair remains puzzling. "Theories cannot really explain why this planet did not fall into the star at some point."
The planet might also help find answers for broader questions in the search for exoplanets. Scientists continue to wonder, for instance, whether an iron and rock composition is the rule or an exception for Earth-sized planets. To help address these questions, NASA has a planned mission in 2017 that will provide better tools for finding more exoplanets and will help put planets like Kepler-78b and Earth in context.