How a moon can reveal a star’s weight

If a star has a planet and that planet has a moon, then scientists can learn about the star.

Washington: In several cases, the weight of a star can be measured directly, but if the star has a planet and that planet has a moon, then scientists can measure their sizes and orbits to learn about the star, according to a new study.

“I often get asked how astronomers weigh stars. We’ve just added a new technique to our toolbox for that purpose,” said Kipping, a predoctoral fellow at the Harvard-Smithsonian Center for Astrophysics.

“Basically, we measure the orbits of the planet around the star and the moon around the planet. Then through Kepler’s Laws of Motion, it’s possible to calculate the mass of the star,” explained Kipping.

By measuring how the star’s light dims when planet and moon transit, astronomers learn three key numbers: the orbital periods of the moon and planet, the size of their orbits relative to the star, and the size of planet and moon relative to the star.

Plugging those numbers into Kepler’s Third Law yields the density of the star and planet. Since density is mass divided by volume, the relative densities and relative sizes give the relative masses.

Finally, scientists measure the star’s wobble due to the planet’s gravitational tug, known as the radial velocity. Combining the measured velocity with the relative masses, they can calculate the mass of the star directly.

“No moon means we can’t work out the exact density of the planet, so the whole thing grinds to a halt,” Kipling said.

Kipping hasn’t put his method into practice yet, since no star is known to have both a planet and moon that transit. However, NASA’s Kepler spacecraft should discover several such systems.


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