Science: Stars and Planets: March

OUR MILKY Way galaxy is populated not just with stars and planets made of ordinary matter, but with worlds made of a strange kind of "mirror matter" that we cannot see. This startling new theory links together experiments in particle physics and measurements of gravitational force in the Universe - and it suggests that aliens could be living much closer to us than we might suspect.

The starting point is evidence that our Galaxy has an unexpectedly strong gravitational pull - more powerful than we would expect from the visible stars and gas making up the Milky Way. It must contain some kind of "dark matter" that has a gravitational pull but is not obvious to any telescope.

Astronomers in Australia and Chile are on the track of this dark matter. They have found that the stars in a neighbouring galaxy, the Large Magellanic Cloud, sometimes seem to brighten unexpectedly. Most likely, their light is being focused by the gravity of objects in the outer part - the halo - of the Milky Way. These objects are called Machos - massive compact halo objects - and they weigh about half as much as the Sun.

Machos cannot be ordinary stars, because they don't emit any detectable amount of light. Most astronomers think that they are some kind of shrunken star - a white dwarf, a neutron star or a black hole. But Rabindra Mohapatra and Vigdor Teplitz, of the University of Maryland have a new idea, rooted in particle physics. There is now a lot of evidence that the light particles called neutrinos can change from one type into another. The results are best fitted if the neutrinos first change into a different kind of matter altogether - so-called "mirror matter" - and then back into another neutrino.

The mirror-world has a counterpart for each of the particles making up the Universe we see, such as the proton and the electron, but - the Maryland physicists calculate - the mirror-particle would weigh only one-twentieth as much. Whereas ordinary heavyweight stars in our Galaxy weigh about 10 times as much as the Sun, the heaviest "mirror stars" would weigh half of the Sun's mass.

Mohapatra and Teplitz realised that is just about the mass of the Machos in our Galaxy. Mirror-stars fit the bill for Machos in another way, too. They would not be visible, because they produce "mirror-light" that can't be detected with an ordinary telescope. .

Mohapatra believes that the mirror-stars could easily have mirror-planets and on these worlds life may have arisen. These aliens cannot see us, any more than we can see them.

What's up this month

Early March is a good time for Mercury-spotting. On March 3, it sets over an hour after sunset at about 7.30pm, before turning tail and plunging back sunwards by mid-month. Its fellow inner planet, Venus, is also visible in sunset skies, growing even brighter and setting three hours after the Sun.

Mars is beginning to put an appearance in our evening skies again, rising about 10pm mid-month. The stars now on view tell us that Spring is on the way. Replacing the winter constellations at centre-stage are the spring constellations of Cancer and Leo - the latter being one of the most ancient star-patterns of all, and one which really does resemble its namesake, the lion.

Heather Couper and