Gassy to the core: UV and X-rays shape exoplanets

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  • Published: Feb 1, 2015
  • Channels: UV/Vis Spectroscopy
thumbnail image: Gassy to the core: UV and X-rays shape exoplanets

This is an exo planet...

Strong irradiation from the host star can cause planets known as mini-Neptunes in the habitable zone to shed their gaseous envelopes and become potentially habitable worlds. Rodrigo Luger/NASA images

Exoplanets resembling the gas giant Neptune might become habitable if the star around which they orbit is of relatively low mass, ultraviolet and X-ray phenomena would shape the planet by removing most of the vast, inhabitable atmosphere leaving behind a rocky core on which life might emerge.

Writing in the journal Astrobiology, Rodrigo Luger and Rory Barnes of the University of Washington, suggest that two phenomena originally thought to preclude life on a distant planet - tidal forces and vigorous stellar activity - might actual give extraterrestrial life a boost if the planet is in orbit around a low-mass star. The team suggests that these two forces might work synergistically to transform uninhabitable "mini Neptunes", which are defined as giant planets with a rocky core lying in the outer orbits of a star system into gas-free, with the possibility of them becoming habitable.

...but has it ceased to be?

Most of the stars in the Milky Way galaxy are of low mass, so-called M dwarfs, much smaller and not as bright as our sun, but with close-in habitable zones. Astrobiologists and astronomers are keen to explore the inner orbits of such stars looking for planets that may well have liquid water and perhaps be habitable. These scientists anticipate that there will be many Earth-like and "super-Earth" planets in these habitable zones around distant stars. Super-Earths are defined being of greater mass than the Earth but smaller than the gas giants of our solar system, such as Neptune and Uranus.

"There are many processes that are negligible on Earth but can affect the habitability of M dwarf planets," Luger explains. "Two important ones are strong tidal effects and vigorous stellar activity." Tidal force is simply the star's gravitational tug on the orbiting planet and how it distorts it periodically from being the near-perfect sphere it would otherwise be. Close-in planets, such as those in the habitable zones of M dwarfs experience much stronger tidal forces than we do on Earth.

Strong tidal forces on a much greater scale than those that give us our twice daily oceanic tides, stretch and distort a planet, causing heating by friction, which drives surface volcanism and can lead to a runaway greenhouse effect that boils away the oceans and all chance of habitability.

Likewise, vigorous stellar activity reduces the chances of ET having a happy life on a planet near an M dwarf star because of the high intensity X-rays and ultraviolet radiation emitted by these stars when they are young. This causes heating of the planet's upper atmosphere and generates eroding winds that lead to the atmosphere being lost to space. Recently, Luger and Barnes demonstrated that these effects could lead to the loss of a planet’s entire surface water too in the first few hundred million years after it forms.

Not so grim

"But things aren't necessarily as grim as they may sound," Luger says. Using computer models, the team has calculated that that tidal forces and atmospheric escape can sometimes shape planets that start out as mini Neptunes into gas-free, potentially habitable worlds. The tidal forces can draw the planet closer to its star where X-rays and UV can boil away vast and chilly hydrogen atmospheres. Left behind in the closer habitable zone orbit is then a hydrogen-free, rocky world.

The team suggests that these "habitable evaporated cores" may well have abundant surface water, as their cores are likely to be rich in water ice which melts. This is one condition that may well make conditions on the planet more suited to the emergence of life. The rate of formation would be critical if hydrogen and helium loss were too slow, then a terrestrial world would not form, too fast and a runaway greenhouse effect would prevail boiling away the oceans. Barnes and Luger note, however, that many other conditions must also be met for such planets to be habitable, such as the evolution of an atmosphere that can generate and recycle nutrients globally. "These evaporated cores are probably lurking out there in the habitable zones of these stars, and many may be discovered in the coming years," Luger adds.

Related Links

Astrobiol, 2015, 15, 57-88: "Habitable Evaporated Cores: Transforming Mini-Neptunes into Super-Earths in the Habitable Zones of M Dwarfs"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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