Discovery of Gabrielle

Gabrielle, a moon of Xena, was discovered on September 10, 2005. The discovery was made using the Laser Guide Star Adaptive Optics (LGS AO) system at the W.M. Keck Observatory. Images were taken at infrared wavelengths (2.1 microns) using the NIRC2 imaging camera over a period of 40 minutes. Gabrielle is located to the right of Xena in the picture.

Xena moves across the sky at a different rate to distant stars, as all planets do. We know that the object is the moon of Xena because it follows Xena across the sky. By contrast, a star would appear to remain fixed. When the images are processed, the background stars appear to streak across the sky while Xena's companion does not. From these observations, we can say that Xena is 60 times brighter than Gabrielle. More observations are planned to calculate the orbit of Gabrielle around Xena, which appears to have an orbital period of about 14 days. From the orbital period and semi-major axis (approximately the average distance between Xena and Gabrielle), it is possible to determine the mass of Xena and confirm that it is more massive than Pluto, as we suspect, and hence a tenth planet. However, it is up to the International Astronomical Union to decide whether to confer planet status to Xena, which depends on the definition they decide upon for what is a planet.

A science article on the discovery has been submitted to the Astrophysical Journal on October 2, 2005 and resubmitted with changes on November 11, 2005. The article can be accessed here. Additional information is also available from Mike Brown's Gabrielle website.

Xena (center) and Gabrielle (right)
Xena and Gabrielle
Gabrielle and Xena The names Xena and Gabrielle are nicknames that we have invented for these objects and are not going to be the official names. They were taken from the TV series Xena: Warrior Princess. The real name for Xena will depend on whether the International Astronomical Union confers planet status to Xena. We will then suggest an official name for Gabrielle!
Laser Guide Star Adaptive Optics
Light from stars is distorted by atmospheric turbulence. These distortions cause stars to appear to twinkle when viewed with the naked eye. Astronomers have been battling the effect of atmospheric turbulence since the advent of the telescope. On a very large telescope like those at Keck Observatory, the atmosphere reduces the level of detail by a factor of 10-100, depending on the wavelength (the color) at which the observations are carried out.

A new technology called adaptive optics (AO) is able to correct for these distortions. At the heart of AO is a deformable mirror that warps like a potato chip. Light is distorted by the atmosphere and undistorted by the deformable mirror, resulting in crisp images of the object being studied. The atmospheric distortions, called the wavefront, are measured as fast as 1000 times per second using a wavefront sensor. The main difficulty is that the wavefront sensor requires a bright star to make the wavefront measurement. Since most of the sky does not contain a sufficiently bright star, astronomers were very limited in the regions of the sky where they could use AO. To avoid this problem, the AO system at Keck Observatory is equipped with a laser guide star (LGS), which is able to create its own star anywhere in the sky!

The Keck Observatory AO system has been used to capture a variety of beautiful images, some of which can be viewed in the AO image gallery. More information about the LGS AO system can be found in the LGS AO website.

Linear scale image of a star with LGS AO off and on

Logarithmic scale image of a star with LGS AO off and on

The Keck LGS in action!

Bright Kuiper Belt Objects

Pluto Easter Bunny Santa
Pluto and Charon Easter Bunny Santa and Rudolph
These images were all taken using the Keck Observatory LGS AO system.

Moons in color The Kuiper Belt is a region in the Solar System beyond the orbit of Neptune. Until the discovery of Xena (68 AU from the sun), Pluto was the biggest known Kuiper Belp Object (KBO). There are four KBOs that are bright enough that they can be imaged with the Keck Observatory LGS AO without relying on a bright enough star being close to the KBO by coincidence. In order of decreasing apparent brightness, they are Pluto, Easter Bunny (2005 FY9), Santa (2003 EL61) and Xena. Pluto has a moon called Charon, which was discovered in 1978. In addition, the discovery of two new, very faint satellites of Pluto was announced on October 31, 2005 by NASA scientists. We found two moons around Santa: the brightest one, at 11 o'clock, is nicknamed Rudolph. On November 29, 2005, we announced the discovery of the second moon, which can be seen directly below the primary KBO. Initially, we believed that it was a background star since it did not appear in all the observations of Santa! However, the orbit of this second satellite around Santa is consistent with the mass estimate for Santa obtained from the orbit of Rudolph.

The Scientists and Engineers
Mike Brown is a professor of astronomy at Caltech who has been on the hunt for KBOs with a great deal of success. In addition to Xena, Easter Bunny and Santa, his program has also uncovered the existence of Sedna, Quaoar and 2004 DW. His collaborators are Chad Trujillo at Gemini Observatory and David Rabinowitz at Yale University. Mike Brown suggested Santa, Easter Bunny and Xena as possible engineering science targets during the commissioning period of the Keck Observatory LGS AO system.

Three Keck Observatory scientists, Antonin Bouchez (now at Caltech), David Le Mignant and Marcos van Dam, searched for and found companions around the KBOs. The remainder of the Keck Observatory LGS AO team that built the instrument is: Peter Wizinowich (lead), Randy Campbell, Jason Chin, Scott Hartman, Erik Johansson, Robert Lafon, David Le Mignant, Paul Stomski and Doug Summers. Additional support was provided by Jim Lyke, Al Conrad, Cynthia Wilburn, Christine Melcher and Ronald Mouser. The LGS was built by Deanna Pennington, Curtis Brown and Pamela Danforth at Lawrence Livermore National Laboratory.

Keck LGS AO Team Keck LGS AO Team
The Keck Observatory LGS AO team celebrates first corrected images with the LGS AO system!

For more information please contact Marcos van Dam:


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