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EXO WORLDS
Habitable Evaporated Cores
by Staff Writers
Seattle WA (SPX) Jan 29, 2015


A habitable evaporated core? Adapted from NASA images.

I recently published a paper in Astrobiology that shows that it is possible to form Earth-mass potentially habitable planets from mini-Neptunes that migrate into the habitable zones of mid- to late M dwarf stars. Click here to see the poster I presented at the March 2014 EBI (Exoplanets, Biosignatures and Instruments) conference in Tucson, AZ. Also, check out the abstract from a talk I gave at AAS.

Low mass M dwarf stars make up nearly three-quarters of the stars in our galaxy. Because they are smaller and dimmer than the Sun, with closer-in habitable zones, they're also the easiest targets for the detection and characterization of habitable planets.

Current and next-generation telescopes are expected to find many Earth and super-Earth planets in the habitable zones of these stars, so it is critical to understand whether these planets are in fact capable of supporting life.

Unfortunately, there are many processes that may negatively affect the habitability of M dwarf planets. Two important ones are strong tidal effects and vigorous stellar activity. The tidal force is a force arising from the differential strength of gravity; put simply, the stellar tug on the near side of the planet (i.e., the side facing the star) is stronger than the tug on the far side (since gravity weakens with distance).

The tidal force can act to stretch out planets, distorting them into ellipsoidal shapes. 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.

Luckily, tidal forces are relatively weak on the Earth, so 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.

When the entire planet is being stretched by tidal forces, friction in the interior dissipates tremendous amounts of energy, which can drive vigorous surface volcanism and in some cases heat up the surface to the point of sending the planet into a runaway greenhouse, during which the oceans boil away (see Barnes et al. 2013).

The second process that can impact the habitability of these planets is vigorous stellar activity. M dwarfs are extremely luminous when young, emitting large amounts of high-energy X-rays and extreme ultraviolet radiation, which can heat the upper atmospheres of their planets, driving strong planetary winds that lead to the partial or complete erosion of their atmospheres.

In a recent paper, Rory Barnes and I showed that most or all of a planet's surface water can be lost due to the stellar activity during the first few hundred million years after formation.

But things aren't necessarily as grim as they may sound. In a paper to be published in the January edition of Astrobiology, we show that tidal forces and stellar activity can actually facilitate the habitability of certain planets around M dwarfs.

Under certain circumstances, tidal evolution and atmospheric escape can shape planets that start out as "mini-Neptunes" (planets larger than the Earth with solid cores and thick hydrogen envelopes, similar to, but smaller than, Neptune) into gas-free, potentially habitable worlds.

Mini-Neptunes typically form far from the star, where cold temperatures mean that water and other molecules are found as ices in the protoplanetary disk. These ices are easily accreted onto the forming planets, along with large amounts of hydrogen and helium gas. These planets thus form as icy/rocky cores surrounded by massive gaseous atmospheres -- they are initially freezing cold, inhospitable worlds.

However, planets need not form and remain in place. Alongside other processes, tidal forces can induce inward planet migration. The energy dissipated by the tides inside the planet has to come from somewhere -- it turns out that it often comes from the orbit of the planet, which shrinks, bringing the planet closer to the star.

This can cause mini-Neptunes to actually migrate into the habitable zone, where planets are exposed to much higher levels of X-ray and ultraviolet radiation. These, in turn, can lead to rapid loss of the hydrogen and helium to space, in some cases leaving behind a hydrogen-free, terrestrial world in the habitable zone, which we call a "habitable evaporated core" (HEC).

Such a planet is likely to have abundant surface water, since its core is rich in water ice -- once in the habitable zone, this ice can melt and form oceans.

It is important to keep in mind that many other conditions must be met in order for the planet to actually be habitable; for instance, it must develop the right kind of atmosphere and be able to sustain certain geochemical cycles necessary for recycling nutrients on a global scale. The timing of the loss of the hydrogen/helium is also crucial.

M dwarfs remain active for many hundreds of millions of years. If you lose your hydrogen too slowly, you might still have a thick gaseous envelope by the time the star ceases to be active, and the resulting planet won't be terrestrial. If you lose the hydrogen too quickly, the surface will be exposed to the detrimental radiation and a runaway greenhouse may be established, resulting in water loss to space and oxidation of the surface.

The bottom line is that this process -- the transformation of a mini-Neptune into an Earth-like world -- could be a new pathway to the formation of habitable worlds around M dwarf stars. Future research will have to address just how likely it is for HECs to actually be habitable. 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.


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