The Next Earth Will Be Radioactive

In the search for extraterrestrial life (and a new Earth), we're always looking for habitable exoplanets. You know, those terrestrial types with liquid water, a temperate climate, and atmosphere that either gave rise to life in the past or could support life in the future, and because it’s what we know, we tend to use the Earth as our model.

But what if we’ve been looking at the problem the wrong way? What if there are better worlds out there, or whole systems with more planets in the habitable zone than just the one like in our system? There might be. A team of geologists and astronomers at Ohio State University might have figured out how to increase the odds of finding a habitable world.

Exoplanet hunters focus their efforts on stars that are about the same size, age, and have a similar composition to our Sun. Again, going with what we know. But not all stars are created equal. Some stars have more or less of certain elements, and that can affect the planets that orbit them.

The Ohio team, so far, has studied eight Sun-like stars. Specifically, they’ve been measuring the amounts of radioactive elements in these stars. Elements like thorium and uranium that are known to play a part in keeping the Earth’s interior warm enough for active plate tectonics.

First, some science. There are radioactive elements like thorium, uranium, and potassium in the Earth's mantle. These elements heat the planet from the inside, but in a different way than does the heat emanating from Earth's core. The Earth’s core started out hot. The radioactive elements in the mantle generate heat as they decay, sort of like the plutonium that’s powering Curiosity on Mars right now. This radioactive heating is crucial. Without it, there wouldn’t be enough heat in the planet for plates tectonics to move. The Earth’s core just can’t provide that kind of heat.

Plate tectonics play an important role in the Earth’s surface water, though it’s a largely mysterious one. Scientists suspect that the same forces of heat convection in the mantle that move Earth's crust also regulate the amount of water in the oceans. That means for a planet to have water for long periods of time – for, say, life to arise –  it needs some kind of crust recycling system. On Earth, that’s convection in the mantle. So it stands to reason that planets rich in radioactive materials like thorium, which is far more energetic than uranium, would be hotter longer, giving life more time to arise.

So coming back to exoplanets and the team from Ohio – they’ve found stars rich in that highly-radioactive element thorium. Seven of the eight stars they’ve studied have higher concentrations of thorium than our Sun. By extension, they reason that the planets around those stars also contain more thorium, which would make them hotter than the Earth. A lot hotter. A star with 2.5 times the amount of thorium in our sun could host a planet that's 25 percent hotter than the Earth. This theoretical planet could have more, and longer, plate tectonics activity, older oceans, and an overall better environment for life.

But the implications are greater still. If these stars host hotter planets overall, there’s a chance we’ve been casting too small a “habitable zone” net. If it turns out that planets around thorium-rich stars are on average significantly warmer than astronomers previously thought, the habitable zone of these star would be larger. And a larger habitable zone would include far more planets.

So why might some stars and their planets have the warm advantage? Scientists thinks it’s just luck of the draw. The elements created in a supernova determine the materials that are available for new stars and planets to form. It might be that that these stars had more thorium available when they formed than our sun did, giving the planets the chemical balance they have. We’re lucky our Sun has the thorium levels it does, but we might be luckier that other stars have higher levels. Better start working on that warp drive.