[meteorite-list] Do Missing Jupiters Mean Massive Comet Belts?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 27 Nov 2012 14:47:11 -0800 (PST)
Message-ID: <201211272247.qARMlBvD013341_at_zagami.jpl.nasa.gov>


Do missing Jupiters mean massive comet belts?
European Space Agency
27 November 2012

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

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 200??C 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.
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 Goran Pilbratt, ESA's Herschel project scientist.
Notes for Editors <http://www.esa.int/SPECIALS/Herschel/SEMYEXDQZ9H_0.html>
Received on Tue 27 Nov 2012 05:47:11 PM PST

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