The Year in Science: Astronomy

Planet Xena, cosmic evolution, gamma ray bursts, and more.

Sunday, January 08, 2006

Planet Xena Rocks The Solar System

For schoolchildren trying to memorize the names and order of the planets, life just got a bit harder. In July astronomer Mike Brown of Caltech announced the discovery of UB313, a body larger than Pluto, orbiting at about three times Pluto's distance from the sun. By Brown's reckoning, the object qualifies as a 10th planet. Some of his colleagues counter that UB313 is more like an outsize asteroid—and many of them want to demote Pluto while they're at it. Either way, the solar system is looking both larger and stranger than we thought.

The confusion began in 1992 with the discovery of the Kuiper belt, a long-theorized second zone of asteroids on the edge of the solar system. Many of the objects there seemed similar to Pluto but smaller. Then Brown spotted UB313, which is about 1,700 miles wide, or about one-quarter bigger than Pluto. If Pluto is a planet, then UB313—which Brown nicknamed Xena—logically should be as well.

By most recent estimates, however, there are probably another two or three objects in the Kuiper belt as large or larger than Pluto, which will further complicate the picture. "Look, I don't care. It's semantics," says Hal Levison, a planetary scientist at Southwest Research Institute's space science department in Colorado. "But if you come up with a strict scientific definition, you either end up with eight planets—Pluto would not make it—or a lot, like more than 20."

Brown favors the eight-planet solution, but he acknowledges that his is not necessarily the most popular view. "The way to make astronomers look stupidest is to declare that Pluto, this thing that's been a planet for 75 years, isn't one," he says. "The bright side is that this is an amazing opportunity to let people see science as a living thing that makes discoveries and changes."

Even Xena's name remains in limbo while scientists scramble for a vocabulary to match their findings. Brown's provisional moniker comes from the canceled television show Xena: Warrior Princess, but a formal name has yet to be certified by the International Astronomical Union because of the putative planet's disputed status (Xena also does not fit the union's naming convention). The added discovery in October that Xena has a moon probably ups its chances of gaining recognition as a planet. "The public tends to think that any self-respecting planet should have a moon. It's just cooler," Brown says. Meanwhile, he has nicknamed the moon Gabrielle—in honor, naturally, of Xena's TV sidekick. —Richard Morgan

Dark Galaxy Sheds Light On Cosmic Evolution

A galaxy without stars seems as nonsensical as a centipede without legs, but last February astronomer Robert Minchin, now at Arecibo Observatory in Puerto Rico, reported the first-ever sighting of just such an object. Minchin views this dark galaxy not as an anomaly but as perhaps a crucially important piece of evidence confirming current theories about how orderly structures—including bright galaxies like our own—emerged from the formlessness of the Big Bang.

Minchin found the new galaxy, VirgoHI 21, when scanning the sky with the 76-meter (249-foot) Lovell radio telescope at the Jodrell Bank Observatory at the University of Manchester in England. The instrument picked up the characteristic radio emissions from hydrogen gas, indicating the presence of a massive swirling cloud. Although the galaxy contains enough hydrogen to give birth to 100 million suns, that never happened, perhaps because nearly all of the galaxy's mass consists of an inert type of material called dark matter. "If you have a low-mass galaxy with a small proportion of ordinary matter and a large proportion of dark matter, it will stay dark and not necessarily form stars," Minchin says. "VirgoHI 21 looks like it's close to the maximum mass for this type of galaxy."

The detection of the first invisible galaxy may settle a long-standing puzzle. For years, computer simulations of cosmic evolution have predicted that the universe should contain far more small galaxies than are actually seen. "If there are lots of smaller dark galaxies out there, that would fill in the hole between theory and observation," Minchin says. To find out how numerous dark galaxies really are, he will soon scan large areas of the sky using the giant 1,000-foot radio telescope at Arecibo. "A single discovery of this type of galaxy will always be controversial, so we're looking to turn up more," Minchin says. —Elizabeth Svoboda

Neutron Stars Implicated In Cosmic Blasts

Neutron stars, stellar corpses that pack the mass of the sun into balls a dozen miles across, are among the most extreme objects in the universe. This year, astronomers found they are also responsible for some of the most powerful explosions—short gamma-ray bursts.

Between May 9 and August 13, NASA's Swift and HETE satellites detected four such bursts and pinpointed their precise locations. Swift then watched the afterglow of all four. The way each burst faded indicated that the likely source of the commotion was either a collision between two orbiting neutron stars or between a neutron star and a black hole. "This is the solution to a 35-year-old cosmic mystery," says Pennsylvania State University astronomer Derek Fox, who helped localize the bursts. "Until these observations, we didn't know where a single burst came from."

Other short gamma-ray bursts have a distinctly different appearance, but they, too, seem connected to neutron stars. Theorists have deduced that they are caused by seismic disturbances in a supermagnetic type of neutron star called a magnetar. In February astronomers released observations showing how intense those disturbances can be: A magnetar explosion in the constellation Sagittarius briefly outshone all the stars in our galaxy in gamma rays, overloading radiation sensors on more than a dozen spacecraft.

"This is the brightest gamma-ray event we've seen," says astrophysicist Steven Boggs of the University of California at Berkeley, who analyzed data from the blast. The star's magnetic field, trillions of times stronger than Earth's, is thought to have triggered an enormous starquake that released more energy than the sun emits in 200 millennia. "The entire crust of the star has shattered," Boggs says. "It's helping us understand how neutron stars are born and evolve." —Alex Stone

Hunt For Another Earth

Is Earth a midget among planets, or are there other worlds out there similar to our own? After a decade of searching for planets orbiting stars like our sun, astronomers had found nothing but giant planets, most of them gas balls like Jupiter, around other stars. But in June a team led by Geoffrey Marcy of the University of California at Berkeley uncovered the best evidence yet that smaller planets are out there—we just weren't able to locate them. Using Hawaii's Keck telescope, he identified a planet only 7.5 times the mass of the Earth circling Gliese 876, a dim red star in the constellation Aquarius.

The new world is most likely composed primarily of rock and ice rather than hydrogen and helium gas, falling between Earth and Uranus in composition. It is an unlikely place to find life, however, because the planet's sun-hugging orbit renders it inhospitably hot.

Most of the 169 known planets have been found using the "wobble" technique. Astronomers like Marcy watch a star to see if it waltzes back and forth, indicating the gravitational tug of an orbiting planet. Improved technology has tripled the precision of this technique, allowing the inference of smaller and smaller planets.

As the search broadens, it is dishing up surprises. In July Caltech planetary scientist Maciej Konacki turned up a world with three suns in the constellation Cygnus. Finding that a planet could exist in a multiple-star system, counter to theoretical expectations, "will put our theories of planet formation to a strict test," says Konacki. In May Canadian astronomers using the Most space telescope deduced that a massive planet had so taken hold of its parent star, Tau Boötis, that one side of the star always faces the planet. The interaction is "unlike anything astronomers have seen before," says star watcher Jaymie Matthews at the University of British Columbia.

Drawing on the latest data, Barrie Jones, Nick Sleep, and David Underwood of the Open University in England calculate that as many as half the star systems in our galaxy known to harbor planets may contain ones that meet the basic requirements for life as we understand it. "In the upcoming 10 years, astronomers will discover the first planets that remind us of home," predicts Marcy, who hopes that Congress will respond by providing funding to examine them. "The search for other Earths is the 21st-century equivalent of the 15th-century's transoceanic sailing voyages. The new worlds we find may ignite an era of exploration not seen since King Ferdinand and Queen Isabella ruled the known cosmos." —Jack Kelley

New and Old Galaxies Show Up in All the Wrong Places

Galaxies, like people, mature at wildly varying rates: Two recent sky surveys show that mature galaxies existed in the early universe and oddly infantile galaxies are hanging around in the present.

The news reflects how little we know about the overall population of the universe. Scientists have cataloged about 10 million galaxies, says George Helou, director of the Infrared Processing and Analysis Center at Caltech. He estimates there are a trillion galaxies in total. And for most of the 10 million, he says, "we only know the basics, where they are and how dim they are." To learn more about specific galaxies, astronomers can draw on NASA's fleet of space telescopes, including Spitzer for infrared, the Hubble Space Telescope for visible light, and the Galaxy Evolution Explorer, called Galex, for ultraviolet.

Spitzer and Hubble have teamed up to observe one of the most distant galaxies ever seen, named HUDF-JD2. Since its light has taken more than 13 billion years to reach Earth, the images come from a time when the universe was less than a billion years old. At that early epoch, cosmologists expected to see mostly small building blocks of today's galaxies. Instead, a team led by Bahram Mobasher of the Space Telescope Science Institute found that HUDF-JD2 is eight times more massive than the Milky Way. "That's remarkable," says Helou. "Until now, we wouldn't think that you could make galaxies emerge that early in the universe."

Galaxy formation also seems to have kept going much later than expected. Galex located three dozen youthful galaxies about 2 billion to 4 billion light-years from Earth. The galaxies appear to have started forming less than 1 billion years ago, much more recently than the Milky Way, which is at least 10 billion years old. "It looks a lot like what you would expect galaxies in the early universe to be like, but we're seeing it in these nearby objects," says Galex principle investigator Chris Martin of Caltech. Such cosmic laggards offer an opportunity to study at close range how most other galaxies might have formed, long ago and far away. —Alex Hutchinson

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