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Some Super-Earths Form in Super Snowstorms


Oct. 16, 2006 — How do “super-Earths” — planets five to 15 times Earth’s size — form around dim stars known as red dwarfs?


Earth and other rocky planets formed in the warmer zone closer to the sun, while gaseous and icy planets formed in the cold farther from the sun.


Now, University of Utah physicist Ben Bromley and colleagues elsewhere have performed calculations showing that for a super-Earth to form near a dim, cold red dwarf star, a “cosmic snowstorm” must occur so that the planet’s rocky core picks up a lot of cold, sticky snow and ice.


Below is a Harvard-Smithsonian Center for Astrophysics news release about the study.


Here are some comments from Bromley, a professor of physics:


“Our work was theoretical, involving planet formation around low-mass stars,” he says. “We calculated when and where planets form, given how gas, dust and icy grains — the raw materials for planets — are apparently distributed around a low-mass star.”


“The calculations are straightforward in the planet formation business, and we have confirmed the basic picture with our own numerical simulations. The new feature is that small stars noticeably change as they age. They start off being fairly bright, but then dramatically dim with time. They initially warm a large region of space, but as they fade, the warm region shrinks around the star. In the colder climates, icy particles condense, in a cosmic snowstorm.”


Bromley says that the study’s calculations are the first to “include the possibility that rocky cores [of planets] forming in warmer regions can, as the star fades, pick up a huge amount of mass in the form of sticky ‘snow.’ Hence we get super-Earths.”


“Timing, of course, is everything, and the calculations show that the rocky cores can grow up in time to enjoy the snow showers. The biggest super-Earths are expected right where they have been observed by planet searches,” Bromley says.


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NEWS RELEASE FROM THE HARVARD-SMITHSONIAN CENTER FOR ASTROPHYSICS


— David Aguilar, 617-495-7462, daguilar@cfa.harvard.edu
— Christine Pulliam, 617-495-7463, cpulliam@cfa.harvard.edu


CfA Release No.: 06-27


“We believe that some super-Earths form during a cosmic ‘snowstorm.'”


– Scott Kenyon, Smithsonian Astrophysical Observatory


SOME SUPER-EARTHS FORM IN SUPER SNOWSTORMS


Cambridge, MA – The 200 known planets that orbit other stars exhibit incredible variety. Among them are a handful of worlds that weigh between five and 15 times Earth. Astronomers believe these “super-Earths” are rocky iceballs rather than gas giants like Jupiter. While theorists can explain how such worlds form around Sun-like stars, the discovery of super-Earths around tiny red dwarf stars was surprising. New research suggests that some super-Earths build up rapidly when local temperatures drop and ices condense out of the surrounding gas.


“We believe that some super-Earths form during a cosmic ‘snowstorm.’ Only this snowstorm envelops the whole planet and lasts millions of years,'” said astronomer Scott Kenyon of the Smithsonian Astrophysical Observatory.


All planets form within a disk of gas and dust surrounding a newborn star.


Rocky planets form close to the star, where it is warm, while icy and gaseous planets form farther out, where it is cold. When it was young, the Sun was relatively stable, leading to a natural progression of small, rocky worlds in the hot inner solar system and large, gaseous worlds in the cold outer solar system.


In contrast, planetary systems around small red dwarf stars undergo dramatic changes in their early history. As the young star evolves, it dims. The warm inner disk starts to freeze, creating conditions where water and other volatile gases condense into snowflakes and ice pellets.


“It’s like a massive cold front that sweeps inward toward the star,”explained first author Grant Kennedy of Mount Stromlo Observatory in Australia. “The ices add mass to a growing planet, and also make it easier for particles to stick together. The two effects combine to produce a planet several times the size of Earth.”


The disks that surround small red dwarf stars tend to contain less material than the disk that formed the solar system. Without the “snowstorms” in these smaller disks, there is not enough material to make super-Earths.


Although astronomers have discovered a few super-Earths orbiting red dwarf stars, it may be tough to find worlds hospitable to humans. All of the known super-Earths are icy worlds with no liquid water. Red dwarf stars are so dim and cool that their warm “habitable zones” are very close to the star, where there is very little planet-forming material.


“It’s difficult to make anything larger than Mercury or Mars in the habitable zone of a red dwarf,” said Kenyon.


The astronomers presented their calculations in a paper authored by Kennedy, Kenyon, and Benjamin Bromley (University of Utah). That paper has been accepted for publication in The Astrophysical Journal Letters and is posted online at http://arxiv.org/abs/astro-ph/0609140


The team now plans to conduct detailed numerical simulations to derive typical timescales for the formation of super-Earths around red dwarf stars.


Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.