Seeking aliens? We need to look for radioactive stuff on exoplanets

Radioactive sludge is probably the last place you would expect to find life (except maybe the Toxic Avenger), but if you’re looking for signs of extraterrestrial life, seek out planets with radioactive elements beneath the surface.

Radioactive anything sounds like the opposite of life-giving. Most life as we know it isn’t going to survive on a planet that could pass for another Chernobyl, though there are exceptions. Disaster zones aside, the amount of long-lived radioactive elements that went into the formation of a rocky planet may determine how habitable it is. Radiogenic heating from thorium and uranium in our planet — and rocky exoplanets like it — internally pushes plate tectonics and acts as one of the forces that power a magnetic field, which helps maintain an atmosphere.

Planets are protected from harsh stellar winds and cosmic radiation by their atmospheres. Mars once had an atmosphere but no magnetic field. What happened there is obvious.

“Thorium and uranium are radioactive and decay to other elements,” scientist Francis Nimmo, who recently led a study published in Astrophysical Journal Letters, told SYFY WIRE. “As they do so, they give off heat, and that is what keeps the Earth warm.”

Earth’s geodynamo generates our magnetic field, which prevents us from turning into Mars. Earth’s liquid outer core experiences convection that creates this dynamo. In the outer core, fluid motion, which is thought to be brought on by heat from radioactive decay, moves hot liquefied iron across a magnetic field that is barely there. This process sparks an electric current that not only creates a magnetic field but also a second magnetic field when it interacts with the radioactive decay-induced motion. Double magnetic fields sustain an atmosphere that keeps us from getting burned.

Heavy elements that heat a planet as they degrade are formed during rare mergers of neutron stars, which are the exposed, super-dense collapsed cores of stars that go supernova. These cores are so dense that they can be up to twice the mass of our Sun. The amounts of thorium and uranium in a planet depend on how close it formed to a neturon star merger.

“Neutron star mergers are so dense that the individual particles making up an atom are squeezed very tightly together. During a neutron star merger, the squeezing becomes much harder — so hard that you can build new elements (like U and Th) out of particles,” Nimmo said.

Nimmo and his team created simulations in which they could raise and lower the amounts of U and Th. Too little of these elements could indicate a weak or nonexistent magnetic field. Too much could mean intense plate tectonics that fuel too much volcanic activity for any hypothetical life-forms to survive, though there are things that live on volcanoes. The problem is that it can be difficult to directly measure how much uranium and thorium is in a star. However, there is a way that can reveal about how much of those elements are hiding deep beneath the surface of a planet.

Spectroscopy can help astronomers figure out whether planets orbiting certain stars have the right amount of radiogenic heating for life, or at least Earthlike life, to thrive. Through the spectra produced by a star, astronomers can determine what chemical interactions happen and the electromagnetic radiation that is emitted on stars. Europium is another element produced during neutron star mergers and detected through spectroscopy.

“We don't expect to see U or Th (they will all be in the crust or the interior) but there might be hints in the atmosphere,” said Nimmo. “For instance, if there's a lot of volcanic activity (which will pollute the atmosphere with things like sulfur), that might be a hint that the planet is very heavily heated."

Because Eu is much easier to measure than uranium or thorium, despite being rare on Earth, Eu measurements obtained through spectroscopy are used as a proxy to determine about how much U and Th are present in stars. That can give an idea of how much of the radioactive elements there are in the planets that orbit these stars, and possibly an idea of which planets in a distant star system could be habitable.

The presence of a magnetic field may or may not mean life is creeping around somewhere on an exoplanet. It may be a hint, but for all we know, there might be life out there beyond anything we can imagine.