From European Southern Observatory
October 26, 2019 -- Astronomers using
ESO's SPHERE instrument at the Very Large Telescope (VLT) have revealed that
the asteroid Hygiea could be classified as a dwarf planet. The object is the
fourth largest in the asteroid belt after Ceres, Vesta and Pallas. For the
first time, astronomers have observed Hygiea in sufficiently high resolution to
study its surface and determine its shape and size.
They found that Hygiea is
spherical, potentially taking the crown from Ceres as the smallest dwarf planet
in the Solar System.
A new SPHERE/VLT image of Hygiea, which could be the
Solar System's smallest dwarf planet yet. As an object in the main asteroid
belt, Hygiea satisfies right away three of the four requirements to be
classified as a dwarf planet: it orbits around the Sun, it is not a moon and,
unlike a planet, it has not cleared the neighbourhood around its orbit. The
final requirement is that it have enough mass that its own gravity pulls it
into a roughly spherical shape. This is what VLT observations have now revealed
about Hygiea.
Credit: ESO/P. Vernazza et al./MISTRAL
algorithm (ONERA/CNRS)
As an object in the main asteroid belt,
Hygiea satisfies right away three of the four requirements to be classified as
a dwarf planet: it orbits around the Sun, it is not a moon and, unlike a
planet, it has not cleared the neighbourhood around its orbit. The final
requirement is that it has enough mass for its own gravity to pull it into a
roughly spherical shape. This is what VLT observations have now revealed about
Hygiea.
"Thanks to the unique capability of
the SPHERE instrument on the VLT, which is one of the most powerful imaging
systems in the world, we could resolve Hygiea's shape, which turns out to be
nearly spherical," says lead researcher Pierre Vernazza from the
Laboratoire d'Astrophysique de Marseille in France. "Thanks to these
images, Hygiea may be reclassified as a dwarf planet, so far the smallest in
the Solar System."
The team also used the SPHERE
observations to constrain Hygiea's size, putting its diameter at just over 430
km. Pluto, the most famous of dwarf planets, has a diameter close to 2400 km,
while Ceres is close to 950 km in size.
Surprisingly, the observations also
revealed that Hygiea lacks the very large impact crater that scientists
expected to see on its surface, the team report in the study published today in
Nature Astronomy. Hygiea is the main member of one of the largest
asteroid families, with close to 7000 members that all originated from the same
parent body. Astronomers expected the event that led to the formation of this
numerous family to have left a large, deep mark on Hygiea.
"This result came as a real surprise
as we were expecting the presence of a large impact basin, as is the case on
Vesta," says Vernazza. Although the astronomers observed Hygiea's surface
with a 95% coverage, they could only identify two unambiguous craters.
"Neither of these two craters could have been caused by the impact that
originated the Hygiea family of asteroids whose volume is comparable to that of
a 100 km-sized object. They are too small," explains study co-author
Miroslav Bro of the Astronomical Institute of Charles University in Prague,
Czech Republic.
The team decided to investigate further.
Using numerical simulations, they deduced that Hygiea's spherical shape and
large family of asteroids are likely the result of a major head-on collision
with a large projectile of diameter between 75 and 150 km. Their simulations
show this violent impact, thought to have occurred about 2 billion years ago,
completely shattered the parent body. Once the left-over pieces reassembled,
they gave Hygiea its round shape and thousands of companion asteroids.
"Such a collision between two large bodies in the asteroid belt is unique
in the last 3-4 billion years," says Pavel Ševe?ek, a PhD student at the
Astronomical Institute of Charles University who also participated in the
study.
Studying asteroids in detail has been
possible thanks not only to advances in numerical computation, but also to more
powerful telescopes. "Thanks to the VLT and the new generation
adaptive-optics instrument SPHERE, we are now imaging main belt asteroids with
unprecedented resolution, closing the gap between Earth-based and
interplanetary mission observations," Vernazza concludes.
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