Habitable Planets: Geochemical 'Sweet Spot' Could Refine the Search

It is no secret that more experts than you can count have turned their attention to the stars, searching for planets that would support Earth-like life. The recent discovery of Kepler 452b, the most Earth-like exoplanet ever found, has even sparked a great deal of public interest. However, it's still unclear if any discovery could actually support life. Now a pair of researchers are claiming to have identified a new way to ensure a little more certainty in this exciting hunt.

A Planet Hunter's Handbook (Click here to jump to the new proposed criteria!)

Traditionally, astronomers use state-of-the-art instruments like the planet-hunting Kepler Space Telescope (KST) to search for potentially habitable worlds in our solar system. Specifically, they are looking for celestial bodies orbiting alien stars at just the right distance - not hot close to boil oceans and not too far to ensure they'd stay frozen. Planets found in this place that's just right for water are aptly called "Goldilocks planets" and Kepler has already identified more than 1000 of them.

Researchers have also been looking closely at the density* of these promising exoplanets, measuring the 'wobble' they experience as their gravitation pull interacts with their host star. Accurate assessments of size and mass, it's been argued, helps researchers see if a planet is actually a terrestrial world (like Earth). The tilt of that orbit - called a planet's obliquity - also plays a role, where an extreme tilt could actually help planets boast flowing water even at the furthest edges of a habitable zone. (Scroll to read on...)

Still, even with all these criteria, the search for habitable worlds remains very unrefined. It is suspected that there are actually billions of potentially habitable worlds just waiting to be found throughout the Milky Way Galaxy alone, but that doesn't mean they all could boast life. In fact, many experts have estimated that life-supporting planets might be much rarer than currently thought, with the most common goldilocks planets orbiting temperamental red dwarf stars.

Despite the fact that most of these planets would have adequate atmospheric conditions, they would be ravaged by intense solar winds. "The ultimate consequence is that any planet potentially would have its atmosphere stripped over time," researcher Ofer Cohen explained in a past release.

Other planets never existed in the first place, with two promising 'super Earths' confirmed as "blips on the radar" due to unusual solar interference just last year. (Scroll to read on...)

A Tectonic Tell

That's why geochemist Matthew Jackson and planetary scientist Mark Jellinek are proposing that planet hunters add a new and very specific criteria to their search: ongoing tectonic activity.

You can think of plate tectonics almost like planetary sweat. When the Earth tries to cool itself, cold plates sink downward and absorb heat, while volcanoes release heat where plates are spreading apart and forming.

"Whether or not plate tectonics can happen actually depends on whether or not the Earth is too hot or too cold," Jackson explained in a statement. "If it's too hot, plate tectonics seizes up and if it's too cold, it freezes up."

In 2013, Jackson and Jellinek revealed what they believe is be most accurate model for the Earth's own geological composition, with a continental crust that boasts a 30 percent decay of uranium, thorium and potassium levels found in the planet's mantel. It's this natural decay, they say, that allows the Earth to keep itself warm.

In a study recently published in the journal Nature Geoscience, the pair build off this work, arguing that if the Earth's crust boasted a more concentrated composition of those elements, plate tectonics wouldn't be possible. (Scroll to read on...)

"If this is the case, you can end up with a planet that has only one big plate and can become an extreme greenhouse like Venus," Jackson said. "The new compositional model gives Earth a sweet spot of its own where its interior is neither too hot nor too cold - a place that allows our current mode of plate tectonics to operate."

If, as experts suspects, other habitable worlds are formed under similar planet formation scenarios as Earth, looking to planet composition would then be an ideal way to confirm if they too undergo 'healthy' tectonic activity.

"Our hypothesis suggests that among the rocky exoplanets, there's another dial that's important to turn when considering whether a planet is habitable or not: its bulk composition," Jackson said. "Bulk composition determines its uranium, thorium and potassium abundance, which governs its internal radiogenic heating and ultimately dictates whether or not plate tectonics can happen - as well as the amount of volcanism and the release of CO2 from a planet that can occur. These are the variables that determine whether a planet can support a habitable climate."

*Wondering how exactly scientists guess at a planet's composition when it's many light years away? The long and the short of it is that scientists look for ideal ratios of light reflection and density, but you can check out a detailed Astronomy Notes lesson on the subject here.

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