Subscribe free to our newsletters via your
. 24/7 Space News .

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.

Thanks for being here;
We need your help. The SpaceDaily news network continues to grow but revenues have never been harder to maintain.

With the rise of Ad Blockers, and Facebook - our traditional revenue sources via quality network advertising continues to decline. And unlike so many other news sites, we don't have a paywall - with those annoying usernames and passwords.

Our news coverage takes time and effort to publish 365 days a year.

If you find our news sites informative and useful then please consider becoming a regular supporter or for now make a one off contribution.
SpaceDaily Contributor
$5 Billed Once

credit card or paypal
SpaceDaily Monthly Supporter
$5 Billed Monthly

paypal only


Related Links
Rodrigo Luger's Astronomy Page
Lands Beyond Beyond - extra solar planets - news and science
Life Beyond Earth

Comment on this article via your Facebook, Yahoo, AOL, Hotmail login.

Share this article via these popular social media networks DiggDigg RedditReddit GoogleGoogle

Kepler astronomers discover ancient star with 5 Earth-size planets
Ames IA (SPX) Jan 29, 2015
Astronomers poring over four years of data from NASA's Kepler spacecraft have discovered a star that's 11.2 billion years old and has at least five Earth-size planets. "We thus show that Earth-size planets have formed throughout most of the Universe's 13.8-billion-year history, leaving open the possibility for the existence of ancient life in the Galaxy," the astronomers wrote in their pap ... read more

Service Module of Chinese Probe Enters Lunar Orbit

Service module of China's lunar orbiter enters 127-minute orbit

Chinese spacecraft to return to moon's orbit

Russian Company Proposes to Build Lunar Base

Gully patterns document Martian climate cycles

The two faces of Mars

Several Drives This Week Put Opportunity Near Marathon Distance

Helicopter Could be 'Scout' for Mars Rovers

NASA, Boeing, SpaceX Outline Objectives to ISS Flights

Boeing will be first to carry US astronauts to space

Japanese businessman set to resume space tourist training

Sailing spacecraft LightSail to harness power of solar wind

More Astronauts for China

China launches the FY-2 08 meteorological satellite successfully

China's Long March puts satellite in orbit on 200th launch

Countdown to China's new space programs begins

NASA's CATS Installed on ISS by Robotic Handoff

Roscosmos, NASA Still Planning on Sending Men Into Space

Russian Cargo Spacecraft to Supply ISS With Black Caviar

Astronauts' year-long mission will test limits

SpaceX releases animation of heavy-lift Falcon rocket

NASA TV Coverage Reset for Launch of Newest Earth-Observing Mission

Japan delays launch of satellite due to weather

British Satellite to Be Launched by Russian Proton-M Carrier Rocket

Dawn ahead!

Habitable Evaporated Cores

Smaller Gas Giants Could Support Life

Will NASA's TESS Spacecraft Revolutionize Exoplanet Hunting?

Vanguard Delivers Advanced EHF Bus Structure Assembly

Graphene edges can be tailor-made

The laser pulse that gets shorter all by itself

Eyeglasses that turn into sunglasses - at your command

The content herein, unless otherwise known to be public domain, are Copyright 1995-2014 - Space Media Network. All websites are published in Australia and are solely subject to Australian law and governed by Fair Use principals for news reporting and research purposes. AFP, UPI and IANS news wire stories are copyright Agence France-Presse, United Press International and Indo-Asia News Service. ESA news reports are copyright European Space Agency. All NASA sourced material is public domain. Additional copyrights may apply in whole or part to other bona fide parties. Advertising does not imply endorsement, agreement or approval of any opinions, statements or information provided by Space Media Network on any Web page published or hosted by Space Media Network. Privacy Statement All images and articles appearing on Space Media Network have been edited or digitally altered in some way. Any requests to remove copyright material will be acted upon in a timely and appropriate manner. Any attempt to extort money from Space Media Network will be ignored and reported to Australian Law Enforcement Agencies as a potential case of financial fraud involving the use of a telephonic carriage device or postal service.