Thousands of exoplanets, or planets orbiting stars other than the Sun, have been discovered throughout the Milky Way galaxy. Astronomers expect that billions more have yet to be discovered — but between extreme exoplanets that rain rocks and others that orbit around dead stars, are any of these billions of worlds likely to be habitable for life like ours?
While astronomers searching for life in the Universe typically focus on exoplanets located in the habitable zone — the distance from a parent star where it isn’t too hot for liquid water to form, but still close enough that liquid water won’t freeze — this isn’t enough to determine whether or not an exoplanet is truly habitable. Liquid water may be crucial for life on Earth, but not all planets located in a star’s habitable zone will have liquid water, and even those that do may be inhospitable for other reasons.
To narrow the search down, a team of researchers has turned to geology as an added measure to determine whether or not an exoplanet might be suitable for life. The research was led by Brendan Dyck, an assistant professor of geology in the Irving K. Barber Faculty of Science at the University of British Columbia’s Okanagan campus, and published in the Astrophysical Journal Letters.
Plate tectonics could be a key factor in habitability
“Just because a rocky planet can have liquid water doesn’t mean it does,” Dyck explained in a press release. “Take a look in our own solar system. Mars is also within the habitable zone and although it once supported liquid water, it has long since dried up.”
In many cases, a planet’s ability to retain liquid water depends on whether or not it has active plate tectonics: the large-scale motion of a planet’s solid exterior layers across its surface.
Plate tectonics can help modulate and sustain a planet’s atmosphere, because this motion allows gases to be cycled throughout the planet. This in turn helps create the pressures that keep liquid water present on the planet’s surface, which will make the planet more likely to be habitable for life like ours over long periods of time.
One of the main differences between planets that have plate tectonics and planets that don’t is the thickness of their crusts, said Dyck.
The crust refers to a rocky planet’s rigid, outermost layer, which rests atop the hot, semi-solid mantle. The hotter, dense material beneath the mantle is referred to as the planet’s core.
If a planet’s crust is too thick, Dyck explained, the malleable interior layers of the mantle won’t be able to sustain plate tectonics. This could result in the planet losing its atmosphere and oceans over time, meaning that even a planet in the habitable zone — like Mars — may not be able to support life like ours.
Larger cores lead to thinner crusts
By modelling the evolution of rocky planets over time, Dyck and his colleagues found that the thickness of a planet’s crust is related to whether or not the planet’s iron is concentrated primarily in its core. The location of this iron will affect the sizes of each different layer in the planet, which in turn will have an impact on whether or not plate tectonics can occur. A thick, heavy crust, for example, will be more difficult to move and break apart than a thinner counterpart.
“As the planet forms, those with a larger core will form thinner crusts, whereas those with smaller cores form thicker iron-rich crusts like Mars,” Dyck explained.
The amount of iron found in a planet’s core (as opposed to its mantle or crust) likely has to do with how that planet formed and evolved. Similar to a snowball, planets are built up from smaller pieces of material in a protoplanetary disk that clump together over time. They can also migrate to different locations in the disk when they’re forming.
If a planet began forming in an iron-rich location, the core could therefore be expected to hold a large fraction of iron.
Future space missions will shed a new light on habitability
Dyck’s findings will have implications in the search for life in the Universe. While the habitable zone might be a good place to start searching, a planet’s geology can help narrow the search even further.
“While a planet’s orbit may lie within the habitable zone, its early formation history might ultimately render it inhabitable,” said Dyck.
“The good news is that with a foundation in geology, we can work out whether a planet will support surface water before planning future space missions.”
Astronomers predict that hundreds of millions of exoplanets in our galaxy are located in their host stars’ habitable zones. Dyck’s results will help them decide which ones are most likely to host atmospheres, and are therefore the best targets in the search for life.
The upcoming James Webb Space Telescope, or JWST, will be an important tool in this search. By observing a wide range of exoplanets, JWST will offer the perfect opportunity to apply Dyck’s findings.
“One of the goals of the JWST is to investigate the chemical properties of extra-solar planetary systems,” Dyck explained.
“It will be able to measure the amount of iron present in this alien worlds and give us a good idea of what their surfaces may look like and may even offer a hint as to whether they’re home to life.”