Some potentially habitable planets began as gaseous, Neptune-like worlds

January 28, 2015 by Peter Kelley
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. Credit: Rodrigo Luger / NASA images

Two phenomena known to inhibit the potential habitability of planets—tidal forces and vigorous stellar activity—might instead help chances for life on certain planets orbiting low-mass stars, University of Washington astronomers have found.

In a paper published this month in the journal Astrobiology, UW doctoral student Rodrigo Luger and co-author Rory Barnes, research assistant professor, say the two forces could combine to transform uninhabitable "mini-Neptunes"—big planets in outer orbits with solid cores and thick hydrogen atmospheres—into closer-in, gas-free, potentially habitable worlds.

Most of the stars in our galaxy are low-mass stars, also called M dwarfs. Smaller and dimmer than the sun, with close-in habitable zones, they make good targets for finding and studying potentially . Astronomers expect to find many Earthlike and "super-Earth" planets in the habitable zones of these stars in coming years, so it's important to know if they might indeed support life.

Super-Earths are planets greater in mass than our own yet smaller than gas giants such as Neptune and Uranus. The is that swath of space around a star that might allow liquid water on an orbiting rocky planet's surface, perhaps giving life a chance.

"There are many processes that are negligible on Earth but can affect the habitability of M dwarf planets," Luger said. "Two important ones are strong tidal effects and vigorous stellar activity."

A tidal force is a star's gravitational tug on an orbiting planet, and is stronger on the near side of the planet, facing the host star, than on the far side, since gravity weakens with distance. This pulling can stretch a world into an ellipsoidal or egglike shape as well as possibly causing it to migrate closer to its star.

"This is the reason we have ocean tides on Earth, as tidal forces from both the moon and the sun can tug on the oceans, creating a bulge that we experience as a high tide," Luger said. "Luckily, on Earth it's really only the water in the oceans that gets distorted, and only by a few feet. But close-in planets, like those in the habitable zones of M dwarfs, experience much stronger tidal forces."

This stretching causes friction in a planet's interior that gives off huge amounts of energy. This can drive surface volcanism and in some cases even heat the planet into a runaway greenhouse state, boiling away its oceans, and all chance of habitability.

Vigorous stellar activity also can destroy any chance for life on planets orbiting low-mass stars. M dwarfs are very bright when young and emit lots of high-energy X-rays and ultraviolet radiation that can heat a planet's upper atmosphere, spawning strong winds that can erode the atmosphere away entirely. In a recent paper, Luger and Barnes showed that a planet's entire surface water can be lost due to such stellar activity during the first few hundred million years following its formation.

"But things aren't necessarily as grim as they may sound," Luger said. Using computer models, the co-authors found that tidal forces and atmospheric escape can sometimes shape planets that start out as mini-Neptunes into gas-free, potentially habitable worlds.

How does this transformation happen?

Mini-Neptunes typically form far from their host star, with ice molecules joining with hydrogen and helium gases in great quantity to form icy/rocky cores surrounded by massive gaseous atmospheres.

"They are initially freezing cold, inhospitable worlds," Luger said. "But planets need not always remain in place. Alongside other processes, tidal forces can induce inward planet migration." This process can bring mini-Neptunes into their 's habitable zone, where they are exposed to much higher levels of X-ray and ultraviolet radiation.

This can in turn lead to rapid loss of the atmospheric gases to space, sometimes leaving behind a hydrogen-free, rocky world smack dab in the habitable zone. The co-authors call such planets "habitable evaporated cores."

"Such a planet is likely to have abundant surface water, since its core is rich in water ice," Luger said. "Once in the habitable zone, this ice can melt and form oceans," perhaps leading to life.

Barnes and Luger note that many other conditions would have to be met for such to be habitable. One is the development of an atmosphere right for creating and recycling nutrients globally.

Another is simple timing. If hydrogen and helium loss is too slow while a planet is forming, a gaseous envelope would prevail and a rocky, terrestrial world may not form. If the world loses hydrogen too quickly, a runaway greenhouse state could result, with all water lost to space.

"The bottom line is that this process—the transformation of a mini-Neptune into an Earthlike world—could be a pathway to the formation of habitable worlds around M dwarf stars," Luger said.

Will they truly be habitable? That remains for future research to learn, Luger said.

"Either way, these evaporated cores are probably lurking out there in the zones of these stars, and many may be discovered in the coming years."

Explore further: Can life emerge on planets around cooling stars?

More information: Astrobiology, online.liebertpub.com/doi/abs/10.1089/ast.2014.1215
View a poster for the research.

Related Stories

Can life emerge on planets around cooling stars?

November 20, 2012

(Phys.org)—Astronomers find planets in strange places and wonder if they might support life. One such place would be in orbit around a white or brown dwarf. While neither is a star like the sun, both glow and so could be ...

Finding infant earths and potential life just got easier

December 4, 2014

Among the billions and billions of stars in the sky, where should astronomers look for infant Earths where life might develop? New research from Cornell University's Institute for Pale Blue Dots shows where - and when - infant ...

The effect of starlight on the atmospheres of mini-Neptunes

January 19, 2015

Exoplanet surveys have discovered the first planets with sizes between 2 and 3.5 Earth radii—slightly smaller than the size of the planet Neptune in our solar system. These planets, dubbed "mini-Neptunes," have been spotted ...

Recommended for you

Clandestine black hole may represent new population

June 28, 2016

Astronomers have combined data from NASA's Chandra X-ray Observatory, the Hubble Space Telescope and the National Science Foundation's Karl G. Jansky Very Large Array (VLA) to conclude that a peculiar source of radio waves ...

Minor mergers are major drivers of star formation

June 28, 2016

Around half of the star formation in the local Universe arises from minor mergers between galaxies, according to data from the Sloan Digital Sky Survey. The patch of sky called Stripe 82 is observed repeatedly to produce ...

Opal discovered in Antarctic meteorite

June 28, 2016

Planetary scientists have discovered pieces of opal in a meteorite found in Antarctica, a result that demonstrates that meteorites delivered water ice to asteroids early in the history of the solar system. Led by Professor ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

katesisco
1 / 5 (1) Jan 29, 2015
Amazing how close Dr Herndon's theory of our planet Earth's formation is to this theory.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.