Some astronomers say that a planet the size of Mars or Earth could be lurking on the fringes of our solar system. But even the latest space telescopes that launched in 2009 stand little chance of finding such a distant object.
Such a world, if it exists, would probably have an orbit far beyond Pluto or similar dwarf planets in the outer solar system. It would likely resemble a frozen version of Mars or Earth at best, a most unsuitable home for life. And it would not be alone.
"When the solar system's story is finally written, it's much more likely that it will have closer to 900 planets rather than the nine that we grew up with," said Alan Stern, a planetary scientist at the Southwest Research Institute (SwRI) in Boulder, Colo.
Just a handful of those potential discoveries might reach the size of Earth, compared to a swarm of Pluto-sized bodies that Stern and others expect to find.
Each object ? be it termed a planet, dwarf planet or otherwise ? would serve as a frozen time capsule that could reveal much about the early evolution of the solar system. It could even force scientists to once again rethink the definition of a planet, following the controversial downgrading of Pluto to a dwarf planet.
Beyond the belt
Pluto's downfall came in part because astronomers discovered a number of smaller planetary objects in the outer solar system. Dwarf planets such as Eris occupy a cluttered, icy region beyond Neptune known as the Kuiper Belt. But a planet the size of Mars or Earth has not turned up at such range.
"For the Kuiper Belt we can already say there is nothing Earth or Mars sized, as its dynamical effects would be easily seen," said Mike Brown, an astronomer at Caltech who led teams that discovered Eris (and nicknamed it ?Xena? at first) and other dwarf planets.
One of Brown's past dwarf planet discoveries, Sedna, occupies a strange elliptical orbit between the Kuiper Belt and the more distant Oort Cloud ? a possible sign of the gravitational influence of another world as big as Earth, one astronomer proposed. But Brown suspects that such a large object would have been spotted already.
Brown and Stern say that the Oort Cloud represents a more likely prospect for worlds the size of Mars or Earth. The Oort Cloud surrounds our solar system with billions of icy bodies at distances as far out as 50,000 times the distance between the sun and Earth.
"Once you go beyond the Kuiper Belt, to the Sedna region or the Oort Cloud, you can always hide things by putting them farther away," Brown told SPACE.com.
How they got there
Brown noted that any future discovery of larger objects in the outer solar system would either suggest that scientists have the wrong idea of how planets form, or might indicate that the early solar system had more material available than previously suspected.
"More interesting to me, though, is that it would be an entirely new class of large body," Brown said. "We don't have any ice rich planetary-sized bodies in the solar system, so we don't really know what they would be like and how they would work."
Stern has long supported the idea of many planet-sized lurkers in the outer solar system. He referred to computer models that show how medium-sized planets might have formed during the chaotic creation of gas giants such as Jupiter, when swirling smaller pieces clumped together to form larger bodies.
"Giant planets gravitationally cleared out regions between them, with each capable of throwing small and medium size planets into the deep solar system," Stern explained.
Define 'planet' for me
Such small or medium planets hanging out in the distant reaches of the solar system would cast renewed doubt on the 2006 ruling of the International Astronomical Union (IAU). Stern has heavily criticized the IAU's decision, which demoted Pluto in part because of its location in the solar system.
"The IAU is slowly beginning to realize that it has made a great mistake," Stern said. He predicted that the organization would retract its 2006 decision if new planet-sized discoveries emerge in the future.
Brown stood by the IAU decision as a "very clear definition" that has scientific usefulness. But he too acknowledged the likely complications with outer solar system planets the size of Mars or Earth.
"It seems pretty obvious that if something Earth sized is discovered, everyone is going to call it a planet," Brown said. "So then we are back to the drawing board, unfortunately."
A question of when
The prospect of bigger planets farther out may have to wait until scientific detection improves. Stern compared the search with existing space telescopes to "looking at the sky through a soda straw," because most telescopes have extremely narrow views of the sky. Even the most powerful telescopes can only directly spot planetary objects about 10 times farther out than Pluto.
NASA's new WISE spacecraft has a very slim chance of spotting a close-in planet with its all-sky infrared survey, Brown and Stern agreed. But they both have higher hopes for the upcoming Large Synoptic Survey Telescope, which should have the ability to spot Earth-sized objects as far out as perhaps 1,000 astronomical units (AU), where 1 AU is the Earth-sun distance.
That 1,000 AU still falls short of the Oort Cloud's vastness, which occupies a region tens of thousands of AU away. Still, Stern suggested that future space exploration might even reach distant Earth siblings ? a can-do attitude that perhaps reflects his role as principal investigator for NASA's outbound New Horizons probe headed for Pluto.
"The Oort Cloud is the solar system's attic, with all sorts of stuff up there," Stern said. "We just don't have a big enough ladder to go up and look around yet."