[5] SPACE: PLANET SPOTTED LOSING MASS (pp143-146; N&V) Orbiting very close to a Sun-like star, a huge, searing hot planet has been discovered to be rapidly losing mass. It might eventually be stripped entirely of its gas envelope, leaving behind a liquid core of lava.
The discovery was made by noting an extended and fleeing envelope of hydrogen, the first firm detection ever of that basic element in the atmosphere of a planet outside our solar system. Like the tail of a comet, the hydrogen and possibly other substances trail the puffed-up atmosphere, which averages about three times the size of the planet's primary diameter.
At least 10,000 tons of material is lost every second.
The planet, named HD209458b, is just 4.4 million miles (7 million kilometers) from its star, or about 100 times closer than Jupiter is to our Sun and even closer than Mercury is to the Sun. It circles the star every 3.5 days in an improbable configuration astronomers have come to call a "hot Jupiter" orbit.
The new study might explain why planets are never found much closer to their stars than this one: They simply don't survive.
How to destroy a planet
It's not known exactly how the mass loss is proceeding, but several processes are probably at work.
Because of the tight orbit, the star would cause immense tides on the planet that "blow away part of the upper atmosphere," explained Alfred Vidal-Madjar, who led the study out of the Institut d'Astrophysique de Paris. As mass is stripped, the planet loses gravitational prowess, making it easier for more hydrogen to flee.
The planet's atmosphere is thought to be about 18,000 degrees Fahrenheit (10,000 Celsius), which would force the hydrogen to expand in the first place, making it readily available to be blown away by tidal forces. The star's heavy and constant "wind" of charged particles might strip hydrogen, too.
The rate of loss is probably accelerating, Vidal-Madjar told SPACE.com, though more study is needed pin down the struggling world's fate.
"The process could become intense enough to blow off the whole atmosphere," he said, leaving behind "a naked solid or liquid core of about 10 times the Earth mass."
That's the core size theorists figure would have been needed to initially attract the gas that built the planet, whose primary diameter is about 1.3 times that of Jupiter. Theorists are not certain, however, whether a giant planet forms by this so-called core-accretion method or if it
simply collapses out of a knot of hydrogen in the disk of debris around a newborn star.Vidal-Madjar said the core, if one exists and if it is revealed, might be solid but is "more probably liquid or 'lava like.'"
Meanwhile, he estimates that the envelope of departing hydrogen extends in front of the planet the equivalent of the world's primary radius, and extends backward four or five planetary radii.
In addition, there's a fan-shaped tail.
"The atmosphere is heated, the hydrogen escapes the planet's gravitational pull and is pushed away by the starlight, fanning out in a large tail behind the planet -- like that of a comet," said Alain Lecavelier des Etangs, also of the Institut d'Astrophysique.
The study will be detailed in the March 13 issue of the journal Nature.
The whole phenomenon is reminiscent of some binary star systems, in which one star -- often a dense neutron star or black hole -- siphons material from a less massive companion. But nothing like this has ever been witnessed involving a planet.
How the study was done
The planet,
discovered in 1999 by a method that notes the gravitational wobble a planet induces on its star, is one of more than 100 worlds so far detected outside our solar system. Because the planet orbits so close to the star and in a favorable edge-on configuration, as viewed from Earth, astronomers have been able to study it in more detail than any other extrasolar planet. When HD209458b passes in front of its star, called HD209458, it blocks a bit of the starlight reaching Hubble. These so-called
transit events, which occur every 3.5 days with this planet, allowed a different group of astronomers in 2001 to find sodium, marking the first detection ever made of an atmosphere of a planet around another star.Other hot Jupiters have been found in slightly tighter orbits, making a trip around their stars every 3 days. But only one has been found closer than that, even though closer-in worlds would be the easiest to detect. David Charbonneau of Caltech, who worked on the 2001 discovery of HD209458b's atmosphere, said the new study might suggest a reason why.
"The implication is that planets initially located even closer to their stars would not survive long," Charbonneau writes in an analysis of the study for Nature.
Charbonneau said the planet's mass loss may be a trickle or a flood -- the new data are not sharp enough to pin that down. He also said the planet's size and other characteristics had already indicated that it contained hydrogen, which is the most common element in the universe and the chief component of stars. Those qualifications do not diminish the value of the new study, however.
"These new observations are of potentially great importance, since we are learning directly about the majority constituent of the planet," Charbonneau said in an e-mail interview.
Vidal-Madjar said the wispy, at-risk envelope of gas might contain other gases, which more observations could reveal. Other exiting gases, being heavier than hydrogen, would help reveal the planet's true fate. For now, it's not known whether or not the world is doomed to a lifetime shorter than its host star. Vidal-Madjar's team has put in for more Hubble time to search for escaping carbon, oxygen and possibly nitrogen.
Hubble is a joint project of NASA and the European Space Agency. Astronomers from Switzerland and the United States also participated in the study.
The struggling world is about 150 light-years from our own, meaning that what astronomers see of it now is actually how it existed 150 years ago. Its star is not visible to the naked eye but can be seen with binoculars in the constellation of Pegasus.
Map the Location of HD209458 with Starry Night software 
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