Undiscovered Planet Lurking in Our Own Solar System?

The Independent: Up Telescope! Search Begins for Giant New Planet.

If you grew up thinking there were nine planets and were shocked when Pluto was demoted five years ago, get ready for another surprise. There may be nine after all, and Jupiter may not be the largest.

The hunt is on for a gas giant up to four times the mass of Jupiter thought to be lurking in the outer Oort Cloud, the most remote region of the solar system. The orbit of Tyche (pronounced ty-kee), would be 15,000 times farther from the Sun than the Earth's, and 375 times farther than Pluto's, which is why it hasn't been seen so far.

But scientists now believe the proof of its existence has already been gathered by a Nasa space telescope, Wise, and is just waiting to be analysed.

The first tranche of data is to be released in April, and astrophysicists John Matese and Daniel Whitmire from the University of Louisiana at Lafayette think it will reveal Tyche within two years. "If it does, John and I will be doing cartwheels," Professor Whitmire said. "And that's not easy at our age."

Once Tyche has been located, other telescopes could be pointed at it to confirm the discovery.

Whether it would become the new ninth planet would be decided by the International Astronomical Union (IAU). The main argument against is that Tyche probably formed around another star and was later captured by the Sun's gravitational field. The IAU may choose to create a whole new category for Tyche, Professor Matese said.

The IAU would also have the final say about the gas giant's name. To the Greeks, Tyche was the goddess responsible for the destiny of cities. Her name was provisionally chosen in reference to an earlier hypothesis, now largely abandoned, that the Sun might be part of a binary star system with a dim companion, tentatively called Nemesis, that was thought responsible for mass extinctions on Earth. In myth, Tyche was the good sister of Nemesis.

Tyche will almost certainly be made up mostly of hydrogen and helium and will probably have an atmosphere much like Jupiter's, with colourful spots and bands and clouds, Professor Whitmire said. "You'd also expect it to have moons. All the outer planets have them," he added.

What will make it stand out in the Wise data is its temperature, predicted to be around -73C, four or five times warmer than Pluto. "The heat is left over from its formation," Professor Whitmire said. "It takes an object this size a long time to cool off."