3. Cosmic Collision Brings Dark Matter Into View
A violent collision between a pair of massive galaxy clusters 3 billion light-years from Earth has turned up the most direct evidence yet of dark matter, the invisible substance thought to make up 80 percent of the matter in the universe. To catch dark matter in action, astrophysicists at Stanford University's Kavli Institute and the universities of Arizona and Florida compared X-ray images of visible matter in the clusters with a map of the total mass, which they obtained by measuring the degree to which light from distant galaxies was bent as it passed by, a phenomenon known as gravitational lensing.
When the researchers compared the two images, "the two didn't line up," says Kavli Institute astrophysicist Marusa Bradac. "This tells us there must be something there, and that it is dark matter." Because dark matter doesn't interact with regular matter, or with itself, it passes right through everything. So when the two galaxy clusters smashed together at a staggering 10 million miles per hour, visible matter slowed down in the crush of the collision, but dark matter flew through unfettered. "The visible matter created a kind of traffic jam in the middle, whereas dark matter has its own highway," Bradac says. The different locations of the two kinds of matter are seen in the image, where red represents visible matter and blue represents the dark stuff.
If dark matter exists, it should be present on Earth too. "There are dark matter particles around me and you right now," says Bradac. The next big step for physicists is to detect it in a terrestrial laboratory. That effort got a major boost two months after the Kavli crew announced its discovery, when the most sensitive — and recently upgraded — dark matter detector in the world went online. The Cryogenic Dark Matter Search (CDMS), buried half a mile deep in an old Minnesota iron mine to shield it from cosmic rays, searches for collisions between dark-matter particles called WIMPS and ordinary atoms in 19 hockey-puck-size hunks of germanium. The apparatus is tuned to spot heat energy deposited by these rare impacts, known as scattering events, so the temperature of the instrument's underground silicon and germanium detectors is held at a chilly –459.58 degrees Fahrenheit, just above absolute zero.
Physicists could detect dark matter in the next five years, says Bradac, assuming it has the properties scientists expect. CDMS physicist Dan Akerib of Case Western Reserve University is wagering that it does. "If WIMPS are just around the corner, we might be able to see something," he says. "I'm betting 10 years of my life that they are."
10. Pluto Demoted
For such a small planet — sorry, dwarf planet — Pluto caused a big uproar this year. In August a 424-person vote by the International Astronomical Union (IAU) in Prague settled the long-standing debate over how many planets our solar system contains, bumping Pluto and fixing the number at a modest eight. Due to a new requirement that planets dominate their gravitational domains, Pluto now falls under the quaint designation "dwarf planet," which applies to objects orbiting the sun that are not satellites and are "nearly round." The decision was met with responses ranging from praise to protest. (Some perplexed elementary-school teachers even held vigils.) But the brief period of national mourning over Pluto's demotion soon gave way to new debate. Although the IAU's conservative description barred many celestial bodies access to the title of planet, it generated a new set of questions. What constitutes a dwarf planet? And what is instead merely a "small solar system body," the default category for the tens of thousands of asteroids and comets floating around the sun? At least 12 other candidates, including three asteroids and nine ice balls, are already up for dwarf status, with heated dispute and more votes sure to follow. Who knew astronomy could be so contentious?
13. Probe Snaps Baby Picture of the Cosmos
For five years, the Wilkinson Microwave Anisotropy Probe (WMAP) has been mapping cosmic microwave background radiation, the ubiquitous afterglow of the Big Bang. In March, the WMAP team released new data that provide the most detailed snapshot of what the universe was like as a trillionth-of-a-second-old newborn. The new portrait expands on the probe's initial results from 2003, which showed fluctuations in the microwave background caused by events 400,000 years after the Big Bang. Since 2003, WMAP researchers have made more precise energy measurements of the microwaves that allow them to look farther back in time. Led by Charles Bennett of Johns Hopkins University, the team accounted for distortions in the microwaves due to collisions with distant clouds of electrons. As a result, researchers could study signals 100 times weaker than what was previously detectable. The findings suggest that the first stars lit up around 400 million years after the Big Bang, 200 million years later than originally estimated. The analysis also yielded a breakdown of the cosmic makeup: The universe consists of 22 percent dark matter and 74 percent dark energy. Dark energy seems to cause the expansion of the universe to accelerate, while dark matter helps hold galaxies and galaxy clusters together. And what of the other 4 percent? That's plain old matter—the stuff we're made of.
22. Methane Rain Falls Mainly on Titan's Plain
Astronomers hoped the Huygens probe would reveal a global ocean of methane when it parachuted to the surface of Titan, Saturn's largest moon, on January 14, 2005. Instead of landing with a splash, the spacecraft made more of a splat. "It landed in mud," says planetary scientist Christopher McKay of NASA's Ames Research Center. And that mud was wet with methane. Studies have detected methane rain on Titan and more than 20 lakes of methane at the moon's north pole. Alien as this may seem, Titan's atmosphere — rich in nitrogen and organic compounds — may be similar to the atmosphere of early Earth.
On Titan, where temperatures range around –300 degrees Fahrenheit, methane can be solid, liquid, or gas. McKay and his colleagues suspect there is a thin layer of liquid methane clouds beneath a higher, thicker cloud bank composed of methane ice crystals. The lower clouds produce a constant drizzle amounting to about two inches of precipitation a year. "That kind of system, in motion like that, with a contained weather cycle—it's just ideal for a methane-based organism," McKay says. Perhaps Earth is not the only place where creatures slither through the mud.
45. Alien Planets Get Smaller, Fatter, Faster, and Hotter
This year ushered in at least two dozen more planets outside our own solar system, including some of the oddest ones yet.
In January astronomers introduced the least massive extrasolar planet yet discovered. Weighing in at just 5.5 times the Earth's mass, planet OGLE-2005-BLG-390Lb, most likely an ice-covered ball of rock, circles a cool red dwarf star some 21,000 light-years away.
In September another extrasolar planet rewrote the record books. HAT-P-1 is about 1.5 times the width of Jupiter, yet remarkably, only half Jupiter's mass. Less dense than cork, it would float in water.
In October astronomers revealed the fastest known planet, named SWEEPS-10, with a "year" just 10 hours long and a surface temperature of perhaps 3,000 degrees Fahrenheit. If its home star were any hotter, the planet might have simply boiled away.
48. Nearby Universe Mapped
After two decades of research on three continents, astrophysicists have produced the largest and most detailed full-sky map of the nearby universe. The three-dimensional plot, known as the 2MASS Redshift Survey, covers 600 million light-years in all directions, reconstructing the positions and velocities of 25,000 galaxies. One cool detail: Our home galaxy, the Milky Way, and our sister galaxy, Andromeda, move at 1.4 million miles per hour relative to the ubiquitous background energy left over from the Big Bang, a standard frame of reference for astronomers.
51. Ice Volcanoes Seen On Saturnian Moon
Saturn's small moon Enceladus — just 300 miles wide — turned in one of the biggest discoveries from NASA's Cassini probe, now orbiting the ringed planet. Data published in March showed a geyser on Enceladus shooting jets of water and fine icy particles hundreds of miles into space. The jets also contained simple carbon compounds. These findings suggest that liquid water may lurk beneath the moon's surface, adding Enceladus to the list of alien locations that could harbor organic chemistry, if not actual life.
Carolyn Porco, head of the Cassini imaging team at the Cassini Imaging Central Laboratory for Operations in Boulder, Colorado, has determined that ice from Enceladus's plume swirls around Saturn to form one of the planet's signature rings. Nobody knows why Enceladus is so active; the leading theory is that the stretching force of Saturn's gravity has heated the moon's interior more efficiently than expected.
67 Complex Organic Molecules Formed in Outer Space
In the past year astronomers working at the National Radio Astronomy Observatory's Green Bank Telescope have identified eight new molecules that are some of the largest and most complex compounds discovered in space. The finds suggest that elaborate organic chemistry — chemistry that might have helped seed life on Earth — may be widespread throughout our galaxy and beyond.
The reactions that produce these new molecules probably take place on the surfaces of interstellar dust grains, says Jan Hollis of NASA's Goddard Space Flight Center; those surfaces serve as a meeting ground where atoms, electrically charged radicals, and small neutral molecules can combine. Interstellar shockwaves — turbulence that accompanies the birth of stars, for instance — provide the energy to overcome chemical barriers to reactions and to eject newly formed molecules into the surrounding gas. Once in the gas, these molecules rotate and emit the telltale radio signatures detected at Green Bank.
75 Astro-Hotel Launched
There are currently about 300 astronauts in the world and only a single destination to aim for — the International Space Station, host to just three people for six months at a time. To remedy this situation, Bigelow Aerospace last July launched Genesis 1, the first inflatable space station. It was a trial balloon, a 14-foot-long scale model; the real thing will use modules three times as large.
Bigelow Aerospace founder Robert Bigelow, who made a fortune with his Budget Suites of America hotel chain, recognized an opportunity in 2001 when Congress cut funding for NASA's Transit Habitat inflatable dormitory. He licensed the technology expecting that "maybe 50 countries might want to expand their astronaut corps." Last summer's launch proved that a private company can set up a space station — well, a miniature mock-up, at least — cheaper and faster than the government agency.
93 Renegade Planet Pair Defy Explanation
A pair of celestial objects found circling one another 450 light-years from Earth have fed a growing debate over the dividing line between planets and stars. With roughly seven and 14 times the mass of Jupiter, respectively, these bodies seem too small to be stars. Yet they have no parent sun to orbit as planets do. Instead, they travel by themselves through our galaxy. "They really are something in between planets and stars," says University of Toronto astrophysicist Ray Jayawardhana, who discovered the pair.
This is the first known example of a gravitationally bound pair of planetary mass objects, and astronomers can't agree on a name for them. Some refer to them as isolated giants or sub-brown dwarfs. Jayawardhana calls them planemos (for "planetary mass objects").
Astronomers also cannot agree on how they were made. One theory suggests that the planemos were violently ejected from a dense collapsing cloud of dust and gas, but that process would almost surely have separated the pair, Jayawardhana says. Perhaps they formed like binary stars, which grow from a gas cloud that splits in two while collapsing. But Jayawardhana asks, "How do you make such a small clump of gas collapse on itself?" Whatever their origin, the planemos are surrounded by disks of rocky debris, suggesting that they each may be at the center of a whole miniature system of planets circling planets.
96 Strange Swirls Spotted at Venus's Pole
Venus is Earth's near-twin in size and mass, yet bafflingly different in other particulars. Recent images from the European Space Agency's Venus Express spacecraft, sent to study the planet's chokingly thick, sulfur-tinged atmosphere, have only deepened the mystery. Where scientists expected to find a single vortex swirling around the planet's south pole, the probe showed a strange double formation some 1,250 miles across. "Our models are not able to predict these sorts of patterns," says Håkan Svedhem, a project scientist for Venus Express. He suspects the spirals have some connection with Venus's superhigh-speed equatorial winds, which, unlike Earth's hurricanes, stay in place year after year.
100 Saturn Sunburst
In September the Cassini spacecraft captured this extraordinary backlit image of Saturn and its gossamer rings. The sun is directly behind the planet — an alignment not visible from Earth — which allowed astronomers to discover two faint outer hoops, never before seen, and to observe in unprecedented detail the microscopic particles that make up the rings. In this exaggerated-color image, the beams fanning away from the rings are an artifact of the photographic process, just as a personal camera sometimes captures the glare in a snapshot at the beach.