http://www.abc.net.au/science/news/stories/s804743.htm
Dying planet being consumed by its sun
Thursday, 13 March 2003
A planet orbiting close to its Sun-like star is shrinking as its mass
is violently boiled off, French astronomers have announced.
The gas giant, known as HD209458b, is one of more than 100 extrasolar
planets so far discovered orbiting other stars, but it is only one of
eight that orbit extremely close. It is unique among known extrasolar
planets because it passes in front of its sun - as seen from Earth -
allowing astronomers to observe and measure it in more detail.
Dr Alfred Vidal-Madjar, research director of France's Institut
d'Astrophysique de Paris and colleagues used NASA's Hubble Space
Telescope to observe three of these passes and compare them.
Their measurements infer that an extended envelope of hydrogen
surrounds the gas giant planet - which is slightly larger than Jupiter
- and suggest that this is a halo of material being evaporated from
the planet by its parent star, slowly boiling away the planet's mass
over time. The work appears in today's issue of the journal Nature.
Studying the relationship between planets and their suns allows
astronomers to learn more about how our Solar System formed five
billion years ago. But the dynamic evolution process between planets
and their suns is difficult to measure as optical telescopes cannot
directly see extrasolar planets, which are too distant, too dark and
overwhelmed by the brightness of their suns.
The unique path of HD209458b, transiting the face of its parent star
as it does, offers astronomers 'a golden opportunity' to make more
direct observations.
The astronomers detected atomic hydrogen absorption during the three
observed transits in 2001: on 7-8 September, 14-15 September and 20
October, using the Space Telescope Imaging Spectrograph instrument
aboard the Hubble.
Each time the planet moved in front of the star, the astronomers
measured a 15% dimming in the parent star's light. This is surprising
because the planet's size, 1.35 times the size of Jupiter, means it
only blocks 1.5% of the area of the star.
This larger area corresponds to an extended atmosphere with a radius
of 4.3 times that of Jupiter. But it also means the gravity of the
planet is not strong enough to hold onto the hydrogen, and so it
trickles away.
Too close for comfort
"The implication is that planets initially located even closer to
their stars would not survive long," explains Dr David Charbonneau of
California Institute of Technology in accompanying commentary on the
work. The fact that there are no planets sitting closer to their suns
seems to support this theory.
The planet lies exceptionally close to its parent star, and orbits
every three and a half days. There are only seven other extrasolar
planets that have similar orbital periods of between three and four
days. Only recently, a planet has been found that has a shorter period
of 1.2 days.
The temperature on the surface of the evaporating planet is about
2,170°C, which the astronomers argue is the upper limit on the
evaporation rate; planets any closer to their suns may evaporate
completely.
Similar to Jupiter in mass and size, HD209458b is nevertheless 100
times closer to its star than Jupiter is to our Sun (Jupiter is about
778 million km from our Sun). As a result, the shrinking planet's
gravity must battle the intense heat and stellar radiation to retain
its atmosphere.
The astronomers have calculated a minimum escape rate for the hydrogen
from the planet, but this suggests that only a tiny amount - about
0.1% - of the gas giant has been lost over its five billion year
history. They suggest that difficulties encountered with their
measurements may mean that the escape rate could be much faster.
"This raises the question of the lifetime of evaporating extrasolar
planets which may be comparable to the star's lifetime itself," write
the authors. "If so, the so-called 'hot Jupiters' could evolve faster
than their parent star, eventually becoming smaller objects, which
could look like 'hot hydrogen-poor Neptune-mass' planets.
"This evaporation process, more efficient for planets close to their
star, might explain the very few detections of 'hot Jupiters' with
orbiting periods shorter than three days," they conclude.
Danny Kingsley - ABC Science Online
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