Earth-Like Worlds Can Still Become Harsh 'Snowball Planets' Inside the Habitable Zone

Wednesday, 16 May 2018 - 6:55PM
Space
Astronomy
Wednesday, 16 May 2018 - 6:55PM
Earth-Like Worlds Can Still Become Harsh 'Snowball Planets' Inside the Habitable Zone
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NASA
As our search for Earth-like exoplanets (which are simply planets in other solar systems) continues, one of the key things we look for is whether a distant planet is located in the "habitable zone" of its star, where it's not close enough to get fried and not far away enough to freeze.

But that might not be enough. According to a new study about to be published in the Astronomical Journal, a planet that's otherwise just like Earth - inside the habitable zone, rocky composition, a suitable atmosphere, lots of surface water, etc. - can still be thrown into a completely inhospitable "snowball state" under the right, or wrong, conditions. Once this happens, chances of life surviving there dwindle down to nothing.

To keep things simple for now, the team of researchers at the University of Washington restricted their sample to planets orbiting around G dwarf stars, which are yellow dwarfs like our sun. Through computer modeling, they determined that this snowball state, which the researchers use to describe an extreme ice age that covers the planet, can be brought on by changes in a planet's orbital path or its tilt.



While the orbital eccentricity of a planet (whether its orbital path is an oval or circle) was a big factor in whether it's susceptible to becoming a big snowball in space, the more interesting factor seems to be the axial tilt, which can be described in this context as how much a planet wobbles in its orbit.

According to the study's lead author Russell Deitrick, if a highly eccentric planet moves too far away from its star during its orbit, or if its axial tilt is over 35 degrees, the planet can become too cold to support life. In comparison, Earth's tilt is only 23.5 degrees and it wobbles very little, allowing our planet to sustain life over its long history.

Deitrick said the following in a statement from the University of Washington:

Opening quote
"If we have a planet that looks like it might be Earth-like, for example, but modeling shows that its orbit and obliquity oscillate like crazy, another planet might be better for follow-up" with telescopes of the future."
Closing quote


The most practical use of this information is mostly for other astronomers: with the thousands of known exoplanets and even greater number of observable stars, there's only so many resources that can be devoted to finding another Earth-like planet. Knowing which planets to ignore right off the bat can be helpful.

It's sort of the opposite problem to when astronomers thought they'd found an Earth-like exoplanet orbiting the nearby star Proxima Centauri, only to learn that the planet, Proxima b, was likely getting roasted by its star. Freezing isn't that much better, we supposed.
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