When items are light years away and weigh billions of billions of tonnes,
science software offers a means of calculating their mass.
However, until now it has been difficult to accurately calculate the precise weights of different objects in a star system - such as the star itself and any orbiting planets.
Kepler's third law of motion states that the cube of a planet's semi-major orbital axis is proportional to the square of the duration of a single revolution around its star.
Now the Harvard-Smithsonian Center for Astrophysics is working on
science software that can use this to work out the weight of the star, by adding information about the observed planet's moon.
With a third object, the densities of the planet and its star can be obtained and plugged into the software to work out their masses.
David Kipping, the astrophysicist working on the project, says: "If there was no moon, this whole exercise would be impossible.
"No moon means we can't work out the density of the planet, so the whole thing grinds to a halt."
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