Natalie Jones, of Rangi Ruru Girls School, asks :-

What stars, other than the Sun, have planets that we know about and how far are they from Earth?

Dane Kent, an astronomer at the University of Canterbury, responded.

To date (2001), astronomers have found 68 planets orbiting stars other than our own Sun. Sixty-six were discovered by carefully watching the motion of some very specific stars (often referred to as the stellar wobble technique). These stars are all within about 200 light years of Earth. (Light travels 300,000 km in a second and 9500,000,000,000 km per year.)

The names of many brighter stars begin with the letters HD, which stand for Henry Draper, a famous astronomer of the late 19th century. Some of these stars have been found to have more than one planet orbiting them, for example HD 168443, which is 123 light years away, has at least two planets whose masses are 7.7 and 17.2 Jupiter masses.

The other two planets were discovered through luck. One, with star HD 209458, was discovered when its shadow was seen to pass in front of its sun (known as the transit technique). It is approximately 150 light years distant from Earth, has a mass of 0.60 Jupiter masses and an orbital period of only 3.5 days.

The other was discovered when the light from a distant source was focussed as it passed through the strong gravitational field near a large dense body which happens to be inline with the Earth, a technique called microlensing. It has a mass of about three Jupiter masses, an orbital period of about 10 years (approximately the same as our Jupiter), and the event occurred at a distance of 17.9 light years from Earth.

Currently, equipment is not sensitive enough to detect planets much smaller than that of Jupiter. It is hoped that within the next decade this will be achievable. Then we might even be able to find Earth-sized planets in orbits around these stars, and possibly even detect traces of life.