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This artist’s impression shows the
surface of the distant dwarf planet Makemake. This dwarf planet is
about two thirds of the size of Pluto, and travels around the Sun in
a distant path that lies beyond that of Pluto, but closer to the Sun
than Eris, the most massive known dwarf planet in the Solar System.
Makemake was expected to have an atmosphere like Pluto, but this has
now been shown to not be the case.
Credit: ESO/L. Calçada/Nick
Risinger
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Dwarf planet Makemake [1] is about two
thirds of the size of Pluto, and travels around the Sun in a distant
path that lies beyond that of Pluto but closer to the Sun than Eris,
the most massive known dwarf planet in the Solar System (eso1142
-http://www.eso.org/public/news/eso1142/). Previous observations of
chilly Makemake have shown it to be similar to its fellow dwarf
planets, leading some astronomers to expect its atmosphere, if
present, to be similar to that of Pluto. However, the new study now
shows that, like Eris, Makemake is not surrounded by a significant
atmosphere.
"As Makemake passed in front of
the star and blocked it out, the star disappeared and reappeared very
abruptly, rather than fading and brightening gradually. This means
that the little dwarf planet has no significant atmosphere,"
says Jose Luis Ortiz. "It was thought that Makemake had a good
chance of having developed an atmosphere -- that it has no sign of
one at all shows just how much we have yet to learn about these
mysterious bodies. Finding out about Makemake's properties for the
first time is a big step forward in our study of the select club of
icy dwarf planets."
Makemake's lack of moons and its great
distance from us make it difficult to study [4], and what little we
do know about the body is only approximate. The team's new
observations add much more detail to our view of Makemake --
determining its size more accurately, putting constraints on a
possible atmosphere and estimating the dwarf planet's density for the
first time. They have also allowed the astronomers to measure how
much of the Sun's light Makemake's surface reflects -- its albedo
[5]. Makemake's albedo, at about 0.77, is comparable to that of dirty
snow, higher than that of Pluto, but lower than that of Eris.
It was only possible to observe
Makemake in such detail because it passed in front of a star -- an
event known as a stellar occultation. These rare opportunities are
allowing astronomers for the first time to find out a great deal
about the sometimes tenuous and delicate atmospheres around these
distant, but important, members of the Solar System, and providing
very accurate information about their other properties.
Occultations are particularly uncommon
in the case of Makemake, because it moves in an area of the sky with
relatively few stars. Accurately predicting and detecting these rare
events is extremely difficult and the successful observation by a
coordinated observing team, scattered at many sites across South
America, ranks as a major achievement.
"Pluto, Eris and Makemake are
among the larger examples of the numerous icy bodies orbiting far
away from our Sun," says Jose Luis Ortiz. "Our new
observations have greatly improved our knowledge of one of the
biggest, Makemake -- we will be able to use this information as we
explore the intriguing objects in this region of space further."
Notes
[1] Makemake was initially known as
2005 FY9. It was discovered a few days after Easter in March 2005,
earning it the informal nickname of Easterbunny
(http://www.mikebrownsplanets.com/2008/07/whats-in-name-part-2.html).
In July 2008 it was given the official name of Makemake
(http://www.iau.org/public_press/news/detail/iau0806/). Makemake is
the creator of humanity and god of fertility in the myths of the
native people of Easter Island.
Makemake is one of five dwarf planets
so far recognised by the International Astronomical Union. The others
are Ceres, Pluto, Haumea and Eris. Further information about dwarf
planets and planets is available from the International Astronomical
Union (http://www.iau.org/public/pluto/).
[2] Another of the telescopes used in
this observing campaign was an 0.84-metre telescope installed by the
Catolica del Norte University of Chile. This telescope is sited on
Cerro Armazones, the future site of the European Extremely Large
Telescope (E-ELT).
[3] Makemake passed in front of faint
star NOMAD 1181-0235723 (where NOMAD refers to the Naval Observatory
Merged Astrometric Dataset) on 23 April 2011. The team observed this
event using seven different telescopes across Brazil and Chile. The
event only lasted about one minute, so the astronomers took advantage
of a specialised high-speed camera known as ULTRACAM
(eso0520- http://www.eso.org/public/news/eso0520/) and a
high-speed infrared imager named ISAAC to capture the event.
[4] In the case of objects that are
orbited by one or more moons the motions of the moons can be used to
deduce the mass of the object. This was not possible in the case of
Makemake.
[5] The dwarf planet was calculated to
have a geometrical albedo of 0.77 ± 0.03, greater than Pluto's, but
smaller than that of Eris. An albedo of 1 represents a perfectly
reflecting body, and 0 a black surface that does not reflect at all.
The observations, together with previous results, indicate that
Makemake has a density of 1.7 ± 0.3 grams per cubic centimetre,
which in turn allowed the team to infer the shape and appearance of
an oblate spheroid -- a sphere flattened slightly at both poles --
with axes of 1430 ± 9 kilometres and 1502 ± 45 kilometres. Makemake
shows no global Pluto-like atmosphere at a level of one thousandth of
that of Pluto's atmosphere. However, it may have an atmosphere that
only covers part of the surface. Such a local atmosphere, which is
possible in theory, is not excluded by the observations.
More information
This research was presented in a paper
"Albedo and atmospheric constraints of dwarf planet Makemake
from a stellar occultation" to appear in the 22 November 2012
issue of the journal Nature.
The team is composed of J. L. Ortiz
(Instituto de Astrofisica de Andalucia, CSIC, Spain), B. Sicardy
(Observatoire de Paris; CNRS; Universite Pierre et Marie Curie;
Universite Paris Diderot; Institut Universitaire de France), F.
Braga-Ribas (Observatoire de Paris, CNRS, France; Observatorio
Nacional/MCTI, Brazil), A. Alvarez-Candal (European Southern
Observatory, Chile; Instituto de Astrofisica de Andalucia, CSIC,
Spain), E. Lellouch (Observatoire de Paris, CNRS, France), et al.
For the full list of authors and
affiliations please refer to the Nature paper.
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