Planetary scientists at the California Institute of Technology have discovered a new planet in the outskirts of the solar system. The object circles the sun every 288 years compared to Pluto's 248 years. It has a diameter of about 1250 km, just about half that of Pluto. Even so, it is larger than all of the objects in the inner asteroid belt combined and is the largest object in the solar system discovered since Pluto in 1930.

The new planet has provisionaly been named "Quaoar" (pronounced KWAH-o-ar) after the creation force of the Tongva tribe who were the original inhabitants of the Los Angeles basin, where the Caltech campus is located.

Quaoar is located about 6.3 billion kilometers from the Earth (43.6 AU) in a region beyond the orbit of Pluto known as the Kuiper belt. This is the region where comets originate and also where planetary scientists have long expected to eventually find larger planet-shaped objects such as Quaoar. The discovery is by far the largest object found so far in that search.

The orbit of Quaoar is nearly circular, unlike the highly elliptical orbit of Pluto. As a result of the ellipticity of the latters orbit, Pluto does at times move further out from the Sun than Quaoar.

Quaoar is probably an "ice" planet with a compostion much like pristine comets. The "ice" would contain frozen gases such as methane and ammonia. It would be too cold for these to ever evaporate. Over billions of years, ultra violet light from the Sun will have blackened the surface of the ice giving the planet a very low albedo (reflecting power) estimated to be only 10%.

Pluto has a higher albedo of about 50%. Its elliptical orbit brings the planet closer to the Sun than Neptune for a few years, as it was in the late 20th century. At these times the planet is warm enough to evaporate some of the frozen gases on its surface, so exposing fresh "ices" which maintian its high reflectivity.

As a result, Pluto currently has a magnitude about 14, considerably brighter than Quaoar with a magnitude about 18. Thus Quaoar is beyond the reach of most amateur telescopes.

Like Pluto, Quaoar is at present in the constellation Ophiuchus, the two being only about 6° apart.

The discovery has been announced of yet another tiny planet beyond Neptune, second only to Pluto in size. See also Quaoar and 2004 DW.

The object is designated 2003 VB12 and has been unofficially named Sedna after the Inuit goddess of the ocean. It was found further away than all other planets by the recently launched Spitzer Space Telescope.

At present Sedna is 89 times the Earth's distance, that is 89 Astronomical Units (AU), from the Sun. It was noticed as a very faint, very slow-moving 'star' on pictures taken with the 1.2-metre Schmidt camera at Mt Palomar, California in November 2003. It was then found on photos back to 2001 and has been tracked at several observatories during 2004. It is extremely faint, magnitude 21, one-millionth of the brightness of the faintest star the eye can see.

The orbit of Sedna is very elliptical with an eccentricity of about 0.8. At its closest in 2076 it will be 11 billion km from the sun, nearly 75 times Earth's distance (75 AU) close to twice Pluto's average distance of 39 AU. Its greatest distance is between 900 AU and 1000 AU, or 130 to 150 billion km from the sun. It takes approximately 12 000 years to complete one orbit.

From Sedna the Sun would appear like an extremely bright star. Even at its closest, the Sun's apparent diameter would be only about 24" as seen from Sedna. The Sun would look point like while the intensity of radiation from the Sun would be only about one-five thousandth of what it is at the Earth. Even so the Sun would appear 2.5 million times brighter than the next brightest star, Sirius and 100 brighter than the full Moon.

The Sun, as seen from Sedna when at its greatest distance, would look much smaller and much fainter giving about as much light as the Full Moon. It would still appear about 10 000 times brighter than Sirius.

The size of Sedna can be estimated only from its brightness; it is much too small and distant to see any planetary disk. The size estimate depends on assumptions about the reflectivity or whiteness of the object. A small light-coloured planet will appear as bright as a large dark planet. Early indications are that 2003 VB12 is around 1500 km in diameter but with a large uncertainty. The Spitzer Space Telescope, working in the infra-red, was unable to detect the asteroid, showing it can't be more than 1700 km in diameter. To get this in perspective, Pluto, with a diameter of 2400 km, is 1/500th of Earth's mass, or one-fifth a heavy as our moon.

Technically, Sedna has an absolute magnitude (H) of 1.7. The next largest trans-Neptunian, 2004 DW, found in February, has an H of 2.2. Asteroid (50 000) Quaoar, the previous second-largest trans-Neptunian, is 2.6. Pluto's absolute magnitude is -1.0, but it is probably much whiter than the other trans-Neptunian asteroids because its frost layer is renewed at every perihelion.

The leader of the research team that found Sedna, Dr Mike Brown, reports there is some evidence of a satellite orbiting Sedna deduced from its apparent slower than expected rotation. He said it was hoped in the very near future to get some observations from the Hubble Space Telescope to put the quetion to rest.

All three 'second biggest' planets, Quaoar, 2004 DW and Sedna have been discovered by the same team led by Dr Mike Brown of the California Institute of Technology, along with Drs. Chad Trujillo of the Gemini Observatory, Hawaii, and David Rabinowitz of Yale University.

There is a fashion for referring to the next biggest planet after Pluto as 'the 10th planet'. This is misleading since Pluto, the so-called Ninth Planet, is simply the largest, so far, and first discovered, of the cloud of small icy asteroids beyond Neptune. All evidence shows that there are no big planets -- even as small as earth size! -- beyond Neptune.

For more information go to: Spitzer Space Telescope and NASA - Mysterious Sedna and Caltech.edu

2004DW, a large Kuiper Belt Object

With an estimated size of somewhere between 840km and 1,800km, 2004 DW may be larger that Quaoar, which has a diameter between 1,000 and 1,400 km. If 2004 DW is at the upper end of the uncertainty range then it is larger than any other object found circling the Sun since Pluto was discovered in 1930. Pluto has a diameter 2,390km across, while its satellite Charon has a 1,186km diameter, so the new planet could be larger than Charon. However, 2004 DW is not large enough to qualify as a major planet, it is more akin to an asteroid although even the lower limit set to its size would make it nearly as big as the largest asteroid, Ceres, which has a diameter 932 km.

2004 DW is likely to consist of ice and rock. It orbits the Sun with an average distance of about 8,400 million km, some 2,400 million km beyond Pluto which has an average distance from the Sun of just below 6,000 million km.

Since 1992 some 800 bodies have been found in the outer Solar System, five could be larger than 1,000 km across. They are in what is known as the Kuiper Belt. This is a region thought to contain a large number of small worlds of rock and ice. It is thought there are many more more Kuiper Belt objects awaiting discovery.

Adapted from a BBC Email news item, dated 2004 February 19