It was a record-breaking year for astronomy. Astronomers identified the most planet-rich known alien solar system, clocked the fastest flows of matter, and discovered the largest cosmic void. They were also surprised by some celestial oddities, including a star with a tail like a comet and an enigmatic blast of radio waves whose source is still unknown.
Many of 2007's most remarkable findings were about alien solar systems, including a system boasting five planets – the most populous system known besides our own. This is a measure of how far astronomy has come in recent times, considering that just 12 years earlier, not a single planet orbiting another star like our Sun had been discovered.
The Hubble space telescope detected what some scientists interpreted as a sign of water in the atmosphere of an alien planet, though the searingly hot gas giant it was found in is highly unlikely to support life.
Astronomers claimed that another newly discovered planet, called Gliese 581c, was at the right distance from its star for liquid water to exist, possibly allowing for life to arise. Follow-up work suggested that it would be too hot for liquid water after all, but that a more distant planet in the same system might have the right temperature instead.
A survey that counted up planet-forming discs around binary stars suggested that alien worlds with double sunsets are quite common in our galaxy.
Black holes were also a source of fascinating discoveries in 2007. Scientists finally found evidence pointing to the source of incredibly energetic particles that rain down on the Earth from space. New observations suggest these mysterious ultra-high-energy cosmic rays are coming from the vicinity of monster black holes as massive as millions or billions of Suns.
Jets firing out of stars that may be collapsing to form black holes were discovered to be the fastest known flows of matter in the universe, pushing up against the cosmic speed limit at 99.999% the speed of light.
But a team of physicists controversially argued that black holes as we know them may not exist. They used theoretical calculations to argue that collapsing objects stop contracting just before they reach the black hole stage, becoming weird objects dubbed "black stars".
There was good and bad news related to the world's observatories in 2007. The venerable Hubble Space Telescope took some hits this year, when its Advanced Camera for Surveys (ACS), arguably its most important instrument, broke. It will remain hobbled until the next space shuttle servicing mission in late 2008. One of the gyroscopes that helps Hubble maintain its bearings also died this year.
A game of political hot potato had the Arecibo radio telescope, the world's largest, threatened with shutdown because of a lack of funding.
But there was good news for observatories as well. In October, a new radio telescope array dedicated to the search for extraterrestrial intelligence opened in California. And in July, the largest ever optical observatory, called the Great Canary Telescope, opened its eye to the sky.
The dark side
Scientists continued to speculate in 2007 about the two kinds of mysterious dark stuff that appear to dominate our universe.
There were suggestions that dark energy – the mysterious force that appears to be causing the universe to expand at ever greater rates – might be an illusion, caused by the way light travels through a hypothesised "Swiss cheese" distribution of matter in the universe, by differences in the expansion rate from one part of the universe to another, or by the physics of hidden extra dimensions.
Other studies investigated dark matter, the invisible substance that outweighs ordinary matter and is detected by the way its gravity influences ordinary matter. There were suggestions that dark matter may be warmer than expected, based on the wreckage of colliding galaxies and the peculiar chemistry of some stars in our galaxy.
Another study argued that dark matter may have produced some strange objects in the early universe called "dark stars" – enormous clouds of gas powered by the annihilation of dark matter in their cores.
Some downright bizarre things came to light in 2007 as well. The bright star Mira, familiar to sky gazers since ancient times, was discovered to have a comet-like tail extending several light years behind it, a phenomenon never seen before.
Cosmologists struggled to explain a giant void in space 1 billion light years across, which is empty of both ordinary and dark matter. And scientists scratched their heads over a weird radio burp that came from 1.5 billion light years away and just might be the last gasp of an evaporating black hole.
Astronomers also discovered a strange new way for massive stars to die, based on follow-up observations of a supernova called SN 2006gy. The stars can suffer a violent demise related to the production of antimatter in their cores.
Other observations allowed astronomers to spot some of the most distant galaxies ever seen, viewed at a time when the universe was just 500 million years old.