Water-Ice Super-Earths

Aug 27, 2010
An image of the disk of material around the star Fomalhaut, with its planet seen in the insert (taken at two different dates). CfA astronomers doing theoretical modeling of watery super-Earths have found that collisions between such planets will not increase the amount of water on either one. Credit: Hubble Space Telescope, NASA and ESA

A "super-Earth" is a planet around another star (an "exoplanet") whose mass is less than about ten times that of the Earth. Of the 480 or so extrasolar planets now known, most have masses larger than the mass of Jupiter, which is 318 times more massive than the Earth. About two dozen, though, appear to have masses that put them into the super-Earth category. About 70 exoplanets have orbits that happen to be aligned such that the planet passes directly between the star and the Earth (a transit).

This coincidence allows astronomers to determine the ’s radius, and then, knowing the mass, one of the most interesting of all its characteristics: its density. The density reveals whether the planet is primarily rocky, gaseous, or perhaps contains water. is, of course, a key ingredient since water is required for an exoplanet to be Earth-like as well as Earth-sized.

Robert Marcus, Dimitar Sasselov, and Lars Hernquist, three CfA astronomers, together with a colleague, have completed a theoretical study of water-bearing super-Earths (or, if they are cold, ice-bearing super-Earths), basing their models in part on the characteristics of two known water/icy super-Earths and the larger icy bodies in the outer .

Planets form gradually when smaller bodies in a stellar disk collide and stick together, and current models seem to be successful in describing the probability distribution of planets ending up as rocky, water/ice, or otherwise; 75% water/ice is the commonly assumed maximum value. When more mature larger bodies collide, though, they can fragment into smaller bodies as well as produce an even larger planet with some characteristics of both.

The CfA scientists wondered whether collisions between water/icy super-Earths could change this distribution and increase the amount of water on a planet, perhaps even to levels above 75%.

The astronomers started with super-Earths that were made of 50% water/ice and 50% serpentine rock (which itself contains another 15% water). They gave the colliding planets mass ratios of between 1:1 and 4:1, and considered a range of impact types, from head-on to glancing. They found that these giant impacts could not increase the fraction of water/ice above 50%. Either the rocky material also accreted at the same proportion as the water, or else the collision actually stripped water away from the planet.

The new results are important because they provide an upper limit to the amount of water/ice (75%) that astronomers can use in modeling any newly discovered exoplanet, and give added credibility to the new field of exoplanet morphology.

Explore further: Is the universe finite or infinite?

add to favorites email to friend print save as pdf

Related Stories

Water Planets

Jan 22, 2010

Of the roughly 420 extra-solar planets now known, about a dozen are in the newly named category of "super-earths," planets whose masses are in between of two and about fifteen earth-masses.

Scientists discover a nearly Earth-sized planet (Update)

Apr 21, 2009

(PhysOrg.com) -- Exoplanet researcher Michel Mayor announces the discovery of the lightest exoplanet found so far. The planet, "e," in the system Gliese 581, is only about twice the mass of our Earth. The ...

Astronomers spot 'super-Earth' 80 light years away

Jan 08, 2010

US astronomers have detected the second smallest exoplanet ever discovered with a mass just four times heavier than the Earth, adding to a growing number of low-mass planets dubbed "super-Earths."

First Solid Evidence for a Rocky Exoplanet (w/ Video)

Sep 16, 2009

(PhysOrg.com) -- The longest set of HARPS measurements ever made has firmly established the nature of the smallest and fastest-orbiting exoplanet known, CoRoT-7b, revealing its mass as five times that of Earth's. ...

Superior Super Earths

Nov 30, 2009

Super Earths are named for their size, but these planets - which range from about 2 to 10 Earth masses - could be superior to the Earth when it comes to sustaining life. They could also provide an answer to ...

Recommended for you

Is the universe finite or infinite?

Mar 27, 2015

Two possiblities exist: either the Universe is finite and has a size, or it's infinite and goes on forever. Both possibilities have mind-bending implications.

'Teapot' nova begins to wane

Mar 27, 2015

A star, or nova, has appeared in the constellation of Sagittarius and, even though it is now waning, it is still bright enough to be visible in the sky over Perth through binoculars or a telescope.

Dark matter is darker than once thought

Mar 27, 2015

This panel of images represents a study of 72 colliding galaxy clusters conducted by a team of astronomers using NASA's Chandra X-ray Observatory and Hubble Space Telescope. The research sets new limits on ...

Galaxy clusters collide—dark matter still a mystery

Mar 26, 2015

When galaxy clusters collide, their dark matters pass through each other, with very little interaction. Deepening the mystery, a study by scientists at EPFL and the University of Edinburgh challenges the ...

Using 19th century technology to time travel to the stars

Mar 26, 2015

In the late 19th century, astronomers developed the technique of capturing telescopic images of stars and galaxies on glass photographic plates. This allowed them to study the night sky in detail. Over 500,000 ...

User comments : 0

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.