The night sky: Scratch a star with wobbles to find a planet (or two)

The night sky as it appears tonight looking south at 8pm. Click on More Pictures for a larger image.

A new planet with nearly three times the mass of Jupiter has been detected orbiting the bright star, Pollux, one of the Gemini twins.

While the 200 extra-solar planets discovered so far have been around fairly inconspicuous stars, Pollux is the 16th brightest star in the sky and can be easily seen from all places on Earth.

The discovery of planets orbiting stars other than the Sun continues at the rate of nearly one a week. The planet discovered around Pollux has the mass of 2.9 Jupiters and orbits the star in 590 days.

The discovery was made independently by two teams of astronomers using the so-called Doppler method, a method of detecting the tiny wobble in the motion of the parent star caused by the planet.

The Doppler method is the most successful planet detection method but because larger planets cause a bigger wobble, the method tends to find large planets in orbits relatively close to their stars. These systems may not be typical.

Doppler observations need to be carried out on many stars over a period of years with a large telescope. The technology needed to measure such small stellar motions was only developed 15 years ago.

In another development, a team of planet-hunters at Geneva Observatory have announced the discovery of two more planets orbiting the southern hemisphere star Mu Arae, which is known to have two planets.

This is the second normal star to have four confirmed planets, the other being 55 Cancri in Cancer.

The outermost of the Mu Arae planets orbits in 11.5 years and has the mass of 1.8 Jupiters making it the closest match yet for Jupiter (which orbits the Sun every 11.9 years). The closest planet to Mu Arae weighs at least 10 Earths and must be very hot as it is three times closer than Mercury is to the Sun.

The constellation of Ara lies immediately south of the tail of Scorpius and is visible from New Zealand almost all year. Pollux and Castor, the Gemini twins currently rise at 5am and can be seen low in the northeast just before dawn.

In the early evening sky, the giant planet Jupiter is very bright and easily spotted high in the west. Almost directly overhead is the landmark constellation of Scorpius and just to its east is Sagittarius. Crux (the Southern Cross) and the Pointers set in the southwest.

Saturn, Mars, Venus and Mercury are all too close to the Sun to be easily seen.

* Grant Christie is an astronomy researcher who writes a monthly column for the Herald.