50000 Quaoar
) [
1] is a
Trans-Neptunian object orbiting the Sun in the
Edgeworth-Kuiper belt. It was discovered on
June 4,
2002 by astronomers
Chad Trujillo and
Michael Brown at the
California Institute of Technology in
Pasadena, California from images acquired at the Samuel Oschin Telescope at
Palomar Observatory. This discovery was announced on
October 7,
2002, at a meeting of the
American Astronomical Society. The earliest
precovery turned out to be a
May 25,
1954 plate from
Palomar Observatory.
Quaoar is estimated to have a diameter of 1260 ±190 km, which at the time of discovery made it the largest object found in the
solar system since
Pluto and, indeed, then the largest known
minor planet (it was later supplanted by , , ,
Sedna and
Orcus).
Quaoar was the first
trans-Neptunian object to be measured
directly from
Hubble Space Telescope images, using a new sophisticated method (see
Brown's pages for a non-technical description and his paper cited in references for details). Given its distance Quaoar is on the limit of the HST resolution (40
milliarcseconds) and its image is consequently
smeared on a few adjacent pixels. By comparing carefully this image with the images of stars in the background and using a sophisticated model of HST optics (
PSF), Brown and Troujillo were able to find the best fit disk's size which would give a similar
blurred image.This method was recently applied by the same authors to measure the size of the biggest TNO so far: .
Quaoar's volume is somewhat more than all of the
asteroids put together, it is about one tenth the diameter of
Earth, one third the diameter of the
Moon or about half the size of
Pluto. It orbits at about
6 billion kilometres (3.7 billion miles) from the
Sun with an orbital period of
287 years.
The planetoid's name follows
International Astronomical Union rules by naming all
trans-Neptunian objects after
creation deities (see
planetary nomenclature). "
Quaoar" is the name of a creation deity of the
Native American Tongva people, native to the area around
Los Angeles, where the discovery was named. The IAU approved the name Quaoar, making it the official name; it also has the
provisional designation .The
memorable number 50000 is a witness to the exciting race to discover a Pluto-sized object; Quaoar's discovery followed that of
20000 Varuna and was in turn followed by bigger discoveries (see
above).The efforts were finally rewarded with the discovery of an object bigger than Pluto: .
|
Quaoar and Pluto - polar view. |
Quaoar has a more typical planetary orbit than
Pluto does—a near-circular, moderately-inclined (~8°) orbit with a radius of ~43
AU as shown on the polar view (Quaoar's orbit in blue, Pluto's in red,
Neptune in grey). The spheres illustrate the current (April 2006) positions, relative sizes and colours. The
perihelia (q),
aphelia (Q) and the dates of passage are also marked.
Quaoar is classified as a
classical trans-Neptunian object. Its
cold orbit is however unusual for
large classical objects that typically follow more eccentric and more inclined orbits (see
cubewanos for the comparison and the definition of
cold and
hot families).
At 43 AU and a near-circular orbit, unlike Pluto which is in 2:3
orbital resonance with Neptune, Quaoar is not significantly perturbed by Neptune.
The ecliptic view illustrates the comparison of Quaoar' near-circular, moderately-inclined orbit with Pluto's highly inclined (~17°), highly eccentric (e=0.25) orbit.
|
Quaoar and Pluto - ecliptic view. |
Pluto's aphelion is beyond (and below) Quaoar's orbit, so that Pluto is closer to the
Sun than Quaoar at some times of its orbit, and farther at others.
Quaoar is believed to be a mixture of rock and ice, like other
Kuiper Belt Objects; however its very low
albedo (estimated at 0.1 but still much higher than that of
20000 Varuna: 0.04) indicates that the ice has disappeared from its outer layers. The surface is moderately red (similar to Varuna and
28978 Ixion) in addition to being dim, unlike that of larger objects that are brighter (high albedo) and present neutral colour (see
colour comparison).
In 2004, scientists were surprised to find signs of
crystalline
ice on Quaoar, indicating that the temperature rose to at least −160
°C (110 K or −260
°F) sometime in the last ten million years. Speculation began as to what could have caused Quaoar to heat up from its natural temperature of −220 °C (55 K or −360 °F). Some have theorized that a barrage of mini-
meteors may have raised the temperature, but the most discussed theory speculates that
cryovolcanism may be occurring, spurred by the decay of
radioactive elements within Quaoar's core (Jewitt & Luu, 2004).
Since then (2006), crystalline water ice was also found on but present in larger quantities and thought to be responsible for the very high albedo of that object (0.7).
If the
New Horizons mission visits several Kuiper Belt Objects after visiting Pluto in
2015, our knowledge of the surfaces of small KBOs should improve but encounters with large objects seem unlikely.
*
Reprint on Brown's site (pdf)
* PMID 15592406.
Reprint on Jewitt's site (pdf)
*
Quaoar discoverers' webpage*
Orbit simulation from NASA JPL site*
Astronomers Contemplate Icy Volcanoes in Far Places -
New York Times article
*
Quaoar, the newest planet . . . or is it? - article in an Australian science magazine
*
New Planet-Shaped Body Found in Our Solar System - article in National Geographic
*
Volcanism possible on planet-like Quaoar - CNN.com
*
Chilly Quaoar had a warmer past - Nature.com article
*
Crystalline Ice on Kuiper Belt Object (50000) Quaoar - article about crystalline ice on Quaoar
