Scientists have a good idea of where to find habitable planets. A certain number of provisos have to be met: it must have an atmosphere and be in the “Goldilocks zone”, the right distance from its parent star to have water in liquid form.
Whether this is possible depends, to a large degree, on the star and its age, and the size of the planet itself.
This is why the discovery of a “Godzilla” Earth has come as a surprise for planet hunters. Enter Kepler-10c, a planet about 560 light years from Earth in the constellation Draco, where it circles its sun-like star every 45 days.
It’s about 2.3 times larger than Earth, and was one of hundreds of planets discovered by Nasa’s spacecraft Kepler.
Using data from Kepler, scientists can tell how large a planet is – in this case about 29 000km across – but not whether it is rocky or has a firm surface. However, a recent discovery by scientists at the Harvard-Smithsonian Centre for Astrophysics has found that it is 17 times as dense – a bit like comparing a styrofoam ball with a bowling ball.
This means that it is dense and rocky, similar to Earth. “This is the Godzilla of Earths!” said Centre for Astrophysics researcher Dimitar Sasselov, director of the Harvard Origins of Life Initiative. “But unlike the movie monster, Kepler-10c has positive implications for life.”
Dense rocky planet
Planet hunters were surprised by the discovery of such a dense rocky planet, dubbed a “mega-Earth” because theoreticians say that a planet like this cannot exist.
Another researcher from the centre, David Latham, said: “The standard theory says that a planet first accumulates a core of rocky or dense materials: dust grains, pebbles, rocks … but once the core mass of the planet exceeds 10 times that of Earth, it’s a one-way street: [it is so heavy that] it pulls [the surrounding] gas to itself and makes a gas giant planet.”
Gas giants, such as Jupiter, are not conducive to life, as they generally comprise helium and hydrogen, rather than the heavy elements required for a planet such as Earth. They also do not have a “surface”, rather varying concentrations of gas.
But, perhaps more fundamentally, this Godzilla planet calls much of what we know about the evolution of planets into question.
Scientists at Nasa have estimated the age of the Kepler 10 system at about 11-billion years, about three billion years after the Big Bang. According to common ideas about the formation of rocky planets, Godzilla should not exist.
After the Big Bang, the universe was a swirl of hydrogen and helium, which coalesced into stars. Only once these stars had gone supernova – collapsing in on themselves to form the heavy elements that we associate with rocky planets – and then exploded outwards, could these rocky planets begin to form.
“We thought that you had to wait a few billion years to get rocky planets … [This discovery] pushes back the [time scale] for the possibility of planets where life might evolve,” Latham said.
Kepler-10c shows that the universe was able to form such huge rocks even during a time when heavy elements were scarce. “Finding Kepler-10c tells us that rocky planets could form much earlier than we thought. And if you can make rocks, you can make life,” said Sasselov.
But the researchers do not know whether Godzilla has life and the compact atmosphere required to support it. Latham said that they would have to wait for the next generation of instruments, such as the James Webb Space Telescope or the Square Kilometre Array, to be able to study Kepler-10c in that kind of detail.