Astronomers have found more than 90 planets that cross in front of, or transit, their stars, and by measuring the amount of starlight that's blocked, they can calculate how big the planet is relative to the star. But they can't know exactly how big the planet is, because they can only estimate the size and the mass of the star, scientists at the Harvard-Smithsonian Center for Astrophysics say.

Research by center astrophysicist David Kipping shows that in some cases, the exact weight can be determined, an institute release says.

If the star has a planet, and that planet has a moon, and both of them cross in front of their star, their sizes and orbits -- and the weight of the star -- can be calculated, Kipping says.

"Basically, we measure the orbits of the planet around the star and the moon around the planet," he says. "Then through Kepler's Laws of Motion, it's possible to calculate the mass of the star."

The planet's moon is vital to the process, he says.

"If there was no moon, this whole exercise would be impossible," he says. "No moon means we can't work out the density of the planet, so the whole thing grinds to a halt."

Since no star with both a planet and a moon that transit has been found yet, Kipping hasn't been able to put his method into practice.

NASA's Kepler spacecraft is expected to be able to find several such systems.

"When they're found, we'll be ready to weigh them," he says.

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