50000 Quaoar

50000 Quaoar

Sum of 16 Hubble exposures registered on Quaoar.
Discovery[1] and designation
Discovered by	Chad Trujillo, Michael Brown
Discovery date	2002 Jun 05 10:48:08 PDT on an image taken 2002 June 04 05:41:40 UT
Designations
Pronunciation	/ˈkwɑːwɑr/ KWAH-wahr)[4]
Named after	Quaoar
Alternate name(s)[note 1]	2002 LM60
Minor planet category	Cubewano[2][3]
Orbital characteristics[5]
Epoch May 18, 2008 (JD 2 454 600.5)
Aphelion	6.716 275 Tm (45.286 AU)
Perihelion	6.270 316 Tm (41.928 AU)
Semi-major axis	6.493 296 Tm (43.607 AU)
Eccentricity	0.038 4
Orbital period	105 181.6 d (287.97 a)
Average orbital speed	4.52 km/s
Mean anomaly	284.861°
Inclination	7.988°
Longitude of ascending node	188.893°
Argument of perihelion	148.508°
Satellites	Weywot[6] (~74 km diameter)[7]
Physical characteristics
Dimensions	890 ± 70 km[7] 844+207−190 km (thermal)[8]
Mass	1.6 ± 0.3  × 1021 kg[7] 0.12 Eris masses[7]
Mean density	4.2 ± 1.3 g/cm³[7] 3.5 g/cm³[9] 2.8 g/cm³ (assuming moon highly eccentric)[9]
Equatorial surface gravity	0.276–0.376 m/s²
Equatorial escape velocity	0.523–0.712 km/s
Geometric albedo	0.199 +0.13−0.07 [8]
Temperature	~43 K
Spectral type	(moderately red) B-V=0.94, V-R=0.64[10]
Apparent magnitude	19.3[11]
Absolute magnitude (H)	2.7
↑ Minor Planet Designations

50000 Quaoar is a binary trans-Neptunian object and dwarf planet candidate orbiting the Sun in the Kuiper belt. It was discovered on June 4, 2002 by astronomers Chad Trujillo and Michael Brown at the California Institute of Technology from images acquired at the Samuel Oschin Telescope at Palomar Observatory.

Discovery

The discovery of Quaoar, a magnitude 18.5 object located in the constellation Ophiuchus, was announced on October 7, 2002, at a meeting of the American Astronomical Society. The earliest prediscovery image proved to be a May 25, 1954 plate from Palomar Observatory. It may qualify as a dwarf planet, given its size inferred from direct observation by the Hubble Space Telescope.

Name

Quaoar is named for the Tongva creator god, following International Astronomical Union naming conventions for non-resonant Kuiper belt objects. The Tongva are the native people of the area around Los Angeles, where the discovery of Quaoar was made. Prior to IAU approval of the name, Quaoar went by the provisional designation 2002 LM₆₀. The minor planet number 50000 was not coincidence, but chosen to commemorate a particularly large object found in the search for a Pluto-sized object in the Kuiper belt, parallel to the similarly numbered 20000 Varuna. However, later even larger discoveries were simply numbered according to the order in which their orbits were confirmed.

Size

In 2004, Quaoar was estimated to have a diameter of 1260 ± 190 km,^[12] which at the time of discovery in 2002 made it the largest object found in the solar system since the discovery of Pluto. Quaoar was later supplanted by Eris, Sedna, Haumea, and Makemake. In addition, the subsequently discovered plutino Orcus is about the same size as Quaoar but Quaoar is notably more massive. It is roughly one tenth the diameter of Earth, one third the diameter of the Moon or half the size of Pluto.

Quaoar was the first trans-Neptunian object to be measured directly from Hubble Space Telescope (HST) images, using a new, sophisticated method (see Brown’s pages for a non-technical description and his paper^[12] 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 (point spread function (PSF)), Brown and Trujillo were able to find the best fit disk size which would give a similar blurred image. This method was recently applied by the same authors to measure the size of Eris.

The uncorrected 2004 HST estimates only marginally agree with the 2007 infrared measurements by the Spitzer Space Telescope which suggest a brighter albedo (0.19) and consequently a smaller diameter (844.4 +206.7−189.6 km).^[8] During the 2004 HST observations, little was known about the surface properties of Kuiper belt objects, but we now know that the surface of Quaoar is in many ways similar to those of the icy satellites of Uranus and Neptune.^[7] Adopting a Uranian-satellite limb darkening profile suggests that the 2004 HST size estimate for Quaoar was approximately 40% too large, and that a more proper estimate would be about 900 km.^[7] Using a weighted average of the Spitzer and corrected HST estimates, Quaoar, as of 2010, can be estimated at about 890 ± 70 km in diameter.^[7]

Dwarf planet?

Since Quaoar is a binary object, the mass of system can be calculated from the orbit of the secondary. Quaoar's estimated density of around 4.2 g/cm³ and estimated size of 890 km^[7] suggests that it should qualify as a dwarf planet if the mass required for hydrostatic equilibrium is proven. Mike Brown estimates that rocky bodies around 900 km in diameter relax into hydrostatic equilibrium, and that icy bodies relax into hydrostatic equilibrium around 400 km.^[13] With an estimated mass greater than 1.3 × 10²¹ kg,^[7] Quaoar likely has the mass required (5 × 10²⁰ kg) for being considered a dwarf planet under the 2006 IAU draft definition of a planet.^[14]

Hit-and-run collision

Planetary scientist Erik Asphaug has suggested that Quaoar may have collided with a dwarf planet up to the size of Mars, stripping the lower density mantle from Quaoar, and leaving behind the denser core.^[15] He envisions that Quaoar was originally covered by a mantle of ice that made it 300 to 500 kilometers bigger than it is today, and that it collided with another Kuiper belt body about twice its size — an object roughly the diameter of Pluto, possibly Pluto itself.^[16]

Orbit

The orbit of Quaoar (yellow) and various other cubewanos compared to the orbit of Neptune (blue) and Pluto (pink).

Orbits of Quaoar and Pluto - ecliptic view.

Orbits of Quaoar (blue) and Pluto (red) - polar view.

Quaoar orbits at about 6 billion kilometres (3.7 billion miles) from the Sun with an orbital period of 287 years.

The orbit is near-circular and moderately-inclined at approximately 8°, typical for the population of small classical Kuiper Belt object (KBO) but exceptional among the large KBO. Varuna, Haumea, and Makemake are all on highly inclined, more eccentric orbits.

Quaoar is the largest body that is classified as a cubewano by both the Minor Planet Center^[3] and the Deep Ecliptic Survey.^[2][17]

The polar view compares the near-circular Quaoar's orbit to highly eccentric (e=0.25) orbit of Pluto (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.

At 43 AU and a near-circular orbit, Quaoar is not significantly perturbed by Neptune,^[2] unlike Pluto which is in 2:3 orbital resonance with Neptune. The ecliptic view illustrates the relative inclinations of the orbits of Quaoar and Pluto. Note that 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.

As of 2008, Quaoar is currently only 14 AU^[18] from Pluto making it the closest large body to the Pluto-Charon system. By Kuiper Belt standards this is very close.

Physical characteristics

With a density estimated to be around 4.2 ± 1.3 g/cm³,^[7] Quaoar is believed to be a mixture of mostly rock with some ice and is possibly the densest known object in the Kuiper belt.^[7] Even dwarf planet Haumea is only estimated to have a density of 2.6 g/cm³.^[7] The albedo could be as low as 0.1, which would still be much higher than the lower estimate of 0.04 for Varuna. This may indicate that fresh ice has disappeared from Quaoar's surface. The surface is moderately red, meaning that the object is relatively more reflective in the red and near-infrared than in the blue. 20000 Varuna and 28978 Ixion are also moderately red in the spectral class. Larger KBOs are often much brighter because they are covered in more ice and have a higher albedo, and thus they present a neutral colour (see colour comparison).

Hubble photo used to measure size of Quaoar.

Cryovolcanism

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.^[19]

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.^[20]

Since then (2006), crystalline water ice was also found on Haumea, but present in larger quantities and thought to be responsible for the very high albedo of that object (0.7).^[21]

More precise (2007) observations of Quaoar's near infrared spectrum indicate the presence of small (5%) quantity of (solid) methane and ethane.^[22] Given its boiling point (112 K), methane is a volatile ice at average Quaoar surface temperatures, unlike water ice or ethane (boiling point 185 K). Both models and observations suggest that only a few larger bodies (Pluto, Eris, Makemake) can retain the volatile ices while the dominant population of small TNOs lost them. Quaoar, with only small amounts of methane, appears to be in an intermediary category.^[22]

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.

Satellite

Artist's conception of the moderately red Quaoar and its moon Weywot.

Quaoar has one known satellite, Weywot, formally (50000) Quaoar I Weywot. Its discovery was reported in IAUC 8812 on 22 February 2007.^[6][23] The satellite was found at 0.35 arcsec from Quaoar with an apparent magnitude difference of 5.6.^[24] It orbits at a distance of 14,500 km from the primary and has an orbital eccentricity of about 0.14.^[7] Assuming an equal albedo and density to the primary, the apparent magnitude suggests that the moon has a diameter of about 74 km (1:12 of Quaoar).^[7] Brown believes it is likely to be a collisional fragment of Quaoar, which he speculates lost much of its ice mantle in the process.^[9] Weywot is estimated to only have 1:2000 the mass of Quaoar.^[7]

Brown left the choice of a name up to the Tongva, who chose the sky god Weywot, son of Quaoar.^[25] The name was made official in MPC #67220 published on October 4, 2009.^[26]

References

External links

• Quaoar discoverers' webpage
• Orbital simulation from JPL (Java) / Ephemeris
• Quaoar could have hit a bigger Pluto-sized body at high speeds (Video Credit: Craig Agnor, E. Asphaug)
• Data at Johnston's Archive
• Astronomers Contemplate Icy Volcanoes in Far Places - New York Times article
• Quaoar, the newest planet . . . or is it? - article in an Australian newspaper
• 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
• Cryovolcanism on Charon and other Kuiper Belt Objects