Chuck Bednar for redOrbit.com – Your Universe Online
Planets capable of supporting life could start out as gaseous worlds similar to Neptune, only to be transformed by the combination of two phenomena that individually can inhibit their potential habitability, University of Washington astronomers have discovered in a new study.
Those phenomena, tidal forces and vigorous stellar activity, could combine transform so-called “mini-Neptunes” (large planets in outer orbits with thick hydrogen atmospheres and solid cores) into gas-free and potentially habitable worlds, UW doctoral student Rodrigo Luger and research assistant professor Rory Barnes explain in this month’s issue of the journal Astrobiology.
The majority of the stars in our galaxy are low-mass stars, or M dwarfs, which as smaller and dimmer than our sun and have close-in habitable zones, they explained. These stars are excellent candidates for study in the search for potentially habitable planets, and astronomers expect that they will be able to find several Earth-like planets orbiting these M dwarfs in the years ahead.
Among those candidates are super-Earths, which are planets greater in mass than ours but smaller than gas giants like Neptune and Uranus. The habitable zone is the region of space surrounding a star where liquid water could be found on the surface of a rocky planet, and worlds located in this area (also known as the Goldilocks zone) are capable of supporting life.
Tidal forces
The researchers explain a tidal force is the gravitational pull that a star has on an orbiting planet, and it is stronger on the side of the planet that faces the host star (the near side) than on the side facing away from it (the far side) because gravity weakens with distance. This tug can stretch a planet into an egg-like shape and cause it to move closer to the 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 noted. “Luckily, on Earth it’s really only the water in the oceans that gets distorted, and only by a few feet.”
With close-in planets like those in the habitable zones of M dwarfs, however, the tidal forces are much stronger. This stretching can cause friction in a planet’s interior, which in turn gives off a tremendous amount of energy. This can drive surface volcanism and could even cause a planet to become so hot that it boils away its oceans, becoming completely uninhabitable.
Vigorous stellar activity (we just like saying this)
Vigorous stellar activity can also prevent a planet orbiting a low-mass star from supporting life, the researchers explained. When M dwarfs are young, they are exceptionally bright and emit high amounts of high-energy X-rays and UV radiation. This can cause a planet’s atmosphere to become heated, generating strong winds that can erode it away completely.
In fact, Luger and Barnes have demonstrated that a planet’s entire surface water can be lost due to such stellar activity in the first few hundred million years after its formation. However, Luger said that things are not “as grim as they may sound,” as his team’s computer models have found that these forces can also turn uninhabitable mini-Neptunes into habitable planets.
Mini-Neptunes tend for form far away from their host star, and are initially inhospitable worlds with freezing cold conditions, Luger said. But tidal forces and other processes and cause them to move inward, pulling them into their host star’s habitable zone and exposing them to far higher levels of X-rays and ultraviolet radiation in the process.
This, in turn, can lead to the rapid loss of atmospheric gases to space, which can leave behind what the authors call “habitable evaporated cores” – hydrogen-free, rocky world smack dab in the habitable zone. A planet like this is “likely to have abundant surface water, since its core is rich in water ice,” Luger said.
Welcome to the habitable zone
Once it enters the habitable zone, this ice “can melt and form oceans,” potentially leading to the evolution of life, he added. Before this can happen, however, several other conditions need to be met, including the development of an atmosphere that can create and reuse nutrients globally.
Also, if gas loss is too slow or too fast while a planet is forming, it could prevent this transformation from occurring. If hydrogen and helium loss is too slow, it could keep a rocky terrestrial world from forming. If hydrogen is lost too quickly, it creates runaway greenhouse conditions, causing the surface to become too hot for surface water to exist.
“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, noting that future research would be needed to determine if they could truly support life.
“Either way, these evaporated cores are probably lurking out there in the habitable zones of these stars, and many may be discovered in the coming years,” he added.
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