Hot super-Earths have atmospheres torn away by host stars

Using the nascent art of asteroseismology, scientists have found evidence of what many astronomers long suspected: a class of planets living so close to their stars that the radiation strips them of their atmosphere.

Peter Devine/Courtesy of University of Birmingham
This image shows a planet being stripped by its host star's heat.

Stars, sometimes so beneficent, giving warmth, light and energy, can also be brutal: smothering planets with blistering radiation, and stripping away their very atmospheres.

Astronomers have long hypothesized the existence of a class of planets that live so close to their star that they are fundamentally altered in this way over time, and new research published Monday in Nature Communications finally provides some evidence to support the theory.

Using NASA's Kepler telescope, a nascent branch of astronomy known as asteroseismology, and some complex statistical analysis, researchers were able to point to the existence of these planets – by detecting their absence.

"Stars are big old balls of gas," explains coauthor Guy Davies of the University of Birmingham, United Kingdom, in a Skype interview with The Christian Science Monitor. "The ones we study aren't massively dissimilar to the sun – perhaps up to 1.5 times its mass."

These stars have huge amounts of acoustic energy, trapped by the vast vacuum lying outside, and this sound energy constitutes the basis of asteroseismology.

Think about choir boys and adult tenors, says Dr. Davies.

"If they are hidden behind a curtain in the choir stalls, you cannot see them, you cannot identify them," says Davies. "But when the choir boy sings, you know from the pitch of his voice that he is small. Likewise, when the tenor sings, his loud, deep voice tells you something of his size."

The same is true of stars and their tell-tale sound signatures, he says. But it only became possible to really delve into this world in 2009, with the launch of NASA's Kepler telescope, which Davies describes as "a wonderful piece of kit."

Asteroseismology is more accurate the longer you keep analyzing the same area. With this data, the researchers have been able to study stars in that field of view with greater precision than ever before.

One of the ways in which the scientists manipulated this data was to plot a chart, comparing the amount of radiation various planets were being subjected to by their star, and the radius of those planets.

And as they added the data, their attention was drawn to a specific area of the chart, indicated by a grey box: a section divested of planets, an area that should have been populated by "hot super-Earths," planets a little larger than Earth, but far, far hotter because of the proximity to their star.

The asteroseismologists' conclusion was that the planets were still physically located where one would expect, but they had changed: their radius had altered.

"The high radiation levels have burned off the atmosphere," says Davies. "Radiation from the star has heated up the gaseous environment and it's burned away."

All that remains is the planet's rocky core. The atmosphere, which originally constituted a far bigger proportion of the planet's radius than on Earth, has been blasted away.

But perhaps that "hot super-Earth desert" on the graph was just coincidence? Perhaps there never had been planets of the expected size at the given distance from the stars, and it was all down to chance?

To rule that out, the researchers ran a series of statistical tests, the two most robust of which gave that chance a probability of between 1 in 5,000 and 1 in 10,000. Those figures easily surpass the relevant threshold for statistical significance, known as the three sigma threshold.

So, now that astronomers have this evidence to work with, proving that stars can be "mean little things" stripping "poor little planets" of their atmosphere, what next?

"We haven't found a system where this is actually happening yet - we simply detect an absence of these planets, when they should be there," explains Davies. "What we need to do now is to find one of these planets undergoing this stripping, or just having done it, and look at the dynamic processes."

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