Massive alien 'Super Earths' may be even stranger than previously thought - and could be able to harbour life at extreme pressures.
According to new research, planets which are rocky like Earth, but are much larger, may be protected from harmful radiation by shields of liquid metal.
If true, it would dramatically increase the number of planets on which life may be able to survive.
The idea is that the huge pressures on these massive worlds may be so great as to create a flowing liquid metal (magnesium oxide), which is usually a transparent ceramic when found at the Earth's core.
This shield of metal could create a magnetic field great enough to shield life from harmful rays - presenting a new way that such planets may be able to harbour aliens.
Previously it was assumed that such planets would not have the magnetic fields like Earth, because their greater mass would not allow flowing liquid cores of metal to form.
This kind of dynamic core is what creates Earth's magnetic field, shielding us from the Sun's more harmful particles.
It was therefore assumed life - at least life similar to life on Earth - could not exist on these worlds.
But the newly theorised method might introduce another way that life could be protected. Some Super Earths have been discovered with what is thought to be liquid water on their surface, heightening the possibility that life of some kind may exist there.
To test the theory, scientists fired lasers at magnesium oxide to recreate the pressure at the centre of a Super Earth. They could that at pressures 14 million times that of Earth's surface pressure, and at 50,000 degrees C, the material formed into a liquid metal with a new crystal structure.
"Our results show that the usual assumption that planetary magnetic fields originate exclusively in iron cores is too limiting," R. Stewart McWilliams, a geophysicist at the Carnegie Institution of Washington, told Space.com.
"Magnetic fields might also form within planetary mantles. In fact, this idea has been speculated on for decades, but now we have hard data to show that, indeed, such a 'mantle-dynamo' is plausible."