Published online: 14 June 2007; | doi:10.1038/news070611-10 / http://www.nature.com/news/2007/070611/full/070611-10.html
Dwarf planet found to be heftier than Pluto
Eris is bigger and heavier than our Solar System's 'ninth planet'.Katharine Sanderson
| Eris is heavy - but that doesn't make it a planet. NASA/JPL-Caltech |
|
Whether
you call it a planet or not, Pluto has officially been overtaken by a
more massive planetary object - Eris (previously nicknamed Xena).
Pluto,
traditionally known as the ninth planet of our Solar System, sits in a
giant zone called the Kuiper belt that is filled with asteroids and
many other planetary bodies. The discovery of more and more objects in
this zone - including Eris, in 2005, which was found to be bigger than
Pluto - led astronomers to try to more strictly define what is and
isn't a planet. In 2006, members of the International Astronomical
Union (IAU) at a meeting in Prague, the Czech Republic, voted that
Pluto be reclassified as a 'dwarf planet'. Eris, by these terms, sits
in the same category.
Now
the notion that Eris is a hefty competitor to Pluto has been backed up
with a measurement of the object's mass. Michael Brown, who discovered
the planet, and Emily Schaller, both from the California Institute of
Technology, Pasadena, say that Eris weighs in at 16.6 billion trillion
kilograms. That makes it the most massive dwarf planet seen yet, and
27% more massive than Pluto.
The figure, reported in
Science1,
was calculated using measurements taken by the Hubble Space Telescope
and the Keck Observatory of Eris's moon Dysnomia circling the planet.
From these, Brown and Schaller first worked out that the moon takes
about 16 days to orbit Eris. Kepler's laws of planetary motion, and
models detailing the gravitational pull between two objects, then
reveal the planet's mass.
Wide loadThe
radius of Eris was measured previously by Frank Bertoldi, from the Max
Planck Institute for Radioastronomy in Bonn, Germany. He found it to be
about 3,000 kilometres across - about a third wider than Pluto. But
getting a value for the mass is much more crucial, he says. "It is the
mass that shows how important an object is gravitationally, or from how
many smaller things it has grown and how long that took," he says.
"Knowing the size and the mass is even better since you get the
density, which tells you about the maturity of the object, something
about its history."
Brown
and Schaller estimate Eris's density to be 2.3 grams per cubic
centimetre, which compares well with that of Pluto and other large
Kuiper belt objects.
The
news that Eris is both bigger and heavier than Pluto is unlikely to
help those wishing to see Pluto reinstated as a planet by the IAU.
According to Mark Sykes, director of the Planetary Science Institute in
Tucson, Arizona, the IAU definition of a planet is "silly". He would
reinstate Pluto, and add Eris, plus two other dwarf planets - bringing
our Solar System's total planet count to 12. "Pluto and Eris are more
like the Earth than the Earth is like Jupiter," says Sykes.
The
problematic clause in the IAU definition, he says, is the rule that a
planet can only be classed as such if it has cleared its orbit - which
means that there are no other similar-sized objects in the
neighbourhood of the planet's orbit apart from its own satellites or
other things under that planet's gravitational influence.
"Clearing the orbit tells you nothing about the nature of
the planet," Sykes counters. And it basically makes it impossible for
anything in the Kuiper belt to qualify. "If one accepts the IAU
definition, then even if Eris was ten times as massive as Earth it
would not be a planet," says Sykes.
But
Bertoldi doesn't expect the debate about the planetary status of Pluto,
and now the larger Eris, to be affected by the news that Pluto is the
less weighty of the two. The dwarf planet definition is okay by him: "I
think it is a very good compromise, a very natural way to divide up the
system."
References : Brown M. E. & Schaller E. L. Science,
316
.
1585
(2007).
Pluton perd son rang de planète naine la plus massivePar Laurent Sacco, Futura-Sciences
Sale temps pour Pluton : après avoir été reléguée au rang de planète naine, la voilà qui est détrônée du rang de planète la plus massive dans cette catégorie. La planète naine Eris dans la ceinture de Kuiper est plus massive de 27% d'après les dernières estimations des astronomes. La vision que nous avions du système solaire n'en finit pas d'évoluer !
http://www.futura-sciences.com/fr/sinformer/actualites/news/t/astronomie/d/pluton-perd-son-rang-de-planete-naine-la-plus-massive_12116/
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