Using ESA’s Herschel space observatory,
astronomers have discovered vast comet belts surrounding two nearby planetary
systems known to host only Earth-to-Neptune-mass worlds. The comet reservoirs
could have delivered life-giving oceans to the innermost planets.
In a previous Herschel study, scientists
found that the dusty belt surrounding nearby star Fomalhaut must be maintained
by collisions between comets. In the new Herschel study, two more nearby
planetary systems – GJ 581 and 61 Vir – have been found to host vast amounts of
cometary debris.
Herschel detected the signatures of cold dust
at 200ºC below freezing, in quantities that mean these systems must have at
least 10 times more comets than in our own Solar System’s Kuiper Belt. GJ 581,
or Gliese 581, is a low-mass M dwarf star, the most common type of star in the
Galaxy. Earlier studies have shown that it hosts at least four planets,
including one that resides in the ‘Goldilocks Zone’ – the distance from the
central sun where liquid surface water could exist.
Two planets are confirmed around G-type star
61 Vir, which is just a little less massive than our Sun. The planets in both
systems are known as ‘super-Earths’, covering a range of masses between 2 and
18 times that of Earth. Interestingly, however, there is no evidence for giant
Jupiter- or Saturn-mass planets in either system.
Debris disc around 61 Vir
The gravitational interplay between Jupiter and
Saturn in our own Solar System is thought to have been responsible for
disrupting a once highly populated Kuiper Belt, sending a deluge of comets
towards the inner planets in a cataclysmic event that lasted several million
years.
“The new observations are giving us a clue:
they’re saying that in the Solar System we have giant planets and a relatively
sparse Kuiper Belt, but systems with only low-mass planets often have much
denser Kuiper belts,” says Dr Mark Wyatt from the University of Cambridge, lead
author of the paper focusing on the debris disc around 61 Vir.
“We think that may be because the absence of
a Jupiter in the low-mass planet systems allows them to avoid a dramatic heavy
bombardment event, and instead experience a gradual rain of comets over
billions of years.” “For an older star like GJ 581, which is at least two
billion years old, enough time has elapsed for such a gradual rain of comets to
deliver a sizable amount of water to the innermost planets, which is of
particular importance for the planet residing in the star’s habitable zone,”
adds Dr Jean-Francois Lestrade of the Observatoire de Paris who led the work on
GJ 581.
However, in order to produce the vast amount
of dust seen by Herschel, collisions between the comets are needed, which could
be triggered by a Neptune-sized planet residing close to the disc. “Simulations
show us that the known close-in planets in each of these systems cannot do the
job, but a similarly-sized planet located much further from the star –
currently beyond the reach of current detection campaigns – would be able to
stir the disc to make it dusty and observable,” says Dr Lestrade.
“Herschel is finding a correlation between
the presence of massive debris discs and planetary systems with no
Jupiter-class planets, which offers a clue to our understanding of how
planetary systems form and evolve,” says Göran Pilbratt, ESA’s Herschel project
scientist.
For
further information visit: http://www.esa.int/esaCP/SEMVDXDQZ9H_index_2.html
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