Name? 2010 SO16
Discovered? In images from the WISE infrared survey satellite, launched in 2009.
Orbit? Very Earth-like, say it's discoverers, Apostolos Christou and David Asher, of Armagh Observatory in Northern Ireland, who report their finding in the Monthly Notices of the Royal Astronomical Society. Not only does 2010 SO16 circle the Sun at the same average distance as Earth, but it follows a path so nearly circular that it can't get close to any planet but Earth. (Most asteroids have very elliptical orbits that can cross the paths of multiple planets.)
How close to Earth? The asteroid is "terraphobic," Christou says, and never comes any closer than 50 times the distance between the Earth and the Moon. From Earth, 2010 SO16 appears to have a horseshoe orbit, slowly approaching the planet and then slowly moving away. It traces out this horseshoe path every 175 years. It has likely been following this path for at least the last 250,000 years, Christou and Asher calculate.
How big is it? 2010 SO16 has an estimated diameter of 200 to 400 meters, making it the largest of Earth's four known horseshoe companions.
Where did it come from? There are three theories: (1) It came from the asteroid belt between Mars and Jupiter. If that was the case, the gravitation of the planets would have to have pulled the asteroid into its current orbit, which the scientists think is unlikely. (2) It's a bit of the Moon that somehow escaped the Earth-Moon system. However, there is no explanation for how it would have gotten from the Moon to its present orbit. (3) In the past, scientists have theorized that there are objects that populate Earth's orbit 60 degrees ahead and behind our path (at triangular equilibrium points) and that they may be relics of the formation of the Earth and other planets 4.5 billion years ago. 2010 SO16 may have started out as one of those objects, Christou and Asher say.