The Would-Be 10th Planet

Today, astronomers at the California Institute of Technology announced new measurements of a Pluto-like object in the outer solar system, which under different circumstances could have been identified as the 10th planet. The object, officially named Eris, belongs to the new class of "dwarf planets" that now includes Pluto. Earlier measurements had securely determined that Eris was physically larger than Pluto, which was one factor in the decision to demote Pluto from planethood and create the new category. The new observations show that Eris is also 27 percent more massive than Pluto -- making the would-be 10th planet the currently undisputed king of the dwarf planets.

In 2005, a tiny moon called Dysnomia was discovered in orbit around Eris. Using both the Hubble Space Telescope and the largest telescope in the world on top of Mauna Kea in Hawaii, the scientists measured the position of Dysnomia on six different nights as it traveled around Eris in a 15-day orbit. There is a simple relation between the size of a moon's orbit and the mass of the planet it circles, formulated in the 16th century by Johannes Kepler. This allowed the CalTech astronomers to calculate the mass of Eris directly from the observations.

This discovery adds further support to the reclassification of objects like Pluto and Eris as "dwarf planets" instead of traditional planets like the other 8 in our solar system. Without the new definitions, there would probably already be 8 Pluto-like planets for school children to memorize in the sequence after Neptune, with hundreds of others likely to be discovered in the coming years. Pluto has made its mark in history, but 50 years from now it will be as anonymous as a random chunk of rock in the asteroid belt.

Images of Distant Stars

Last week, astronomers from the University of Michigan announced that they have successfully obtained the first image of a distant Sun-like star. Using a technique known as optical interferometry, the scientists combined the light from a group of four small telescopes scattered across the top of a mountain in California -- effectively creating one huge telescope more than 200 meters across. The resulting image is about 100 times sharper than the view from the Hubble Space Telescope.

Astronomers have been making images of the nearest star (our Sun) since the time of Galileo, but other stars are so far away that even the most powerful telescopes see them as single points of light. In 1995, the Hubble Space Telescope obtained the first direct image of a distant star -- the supergiant Betelgeuse in the constellation Orion, which is so enormous that it would span the orbit of Jupiter in our solar system. The technique of interferometry -- combining the signals from many individual telescopes to produce a sharper image -- has been used in radio astronomy for a long time, most famously in New Mexico at the Very Large Array that was featured in the movie Contact, with Jodie Foster. Recent advances in technology have now made it possible to do a similar thing with optical telescopes.

The target of the University of Michigan study was Altair, the brightest star in the constellation Aquila, which is about 80% more massive than the Sun and spins about 60 times faster. It's rotation is so fast -- more than 600,000 miles per hour at the equator -- that Altair bulges out around the middle. The new image not only reveals this flattened shape, it also shows that the star is much cooler around the equator than near the pole. Although this feature was expected, the predictions do not match the observations exactly -- suggesting that astronomers may need to improve the theory.

There are several arrays of telescopes around the world that are designed for this type of imaging. As the technology continues to improve, imaging of distant Sun-like stars will become routine. This is just the beginning of an exciting era for interferometry.