Star Light, Star Bright, What Do You Weigh Tonight


Computer models of stellar structure are providing estimates of how much a star weighs. It?s a long shot but if the star, like our Sun, has a planet, and the planet has a moon, and both of them cross in front of that star, by measuring their sizes and orbits scientists can tell us about the star. David Kipping, an astrophysicist at the Harvard Smithsonian Center for Astrophysics [CfA] has been working on the formula.
The amount of starlight that?s blocked by planets passing in front of a star is used to determine how large the planet is relative to the star. First, however, scientists need to know the exact size of the star. Computer models are only a good estimate. By using all three heavenly bodies: moon, planet, star ? a moon large enough for us to see it that blocks the starlight, a planet that also blocks starlight, and the star itself ? the triad system can be measured so scientists can figure the size and mass of all three bodies.
Artistic rendering of exoplanet and moon transiting a sun-like star. Credit: David Al Aguilar [CfA]
Artistic rendering of exoplanet and moon transiting a sun-like star. Credit: David Al Aguilar [CfA]
It gets down to Kepler?s Laws of Motion after the orbits of the planet around the star and the moon are determined. The scientists use three key numbers: the orbital periods of the moon and planet, the size of their orbits relative to the star, and the size of the planet and moon relative to the star. Kepler?s Third Law provides the density of the star and planet. Without a moon the planet?s density would remain a secret. A fourth variable in the recipe is the star?s wobble caused by the planet?s gravitational pull, known as radial velocity.
Sadly, after compiling all these calculations and formulas, so far no star has been found that has both a planet and moon that transit. Bets are that NASA?s Kepler spacecraft while searching for Earth-sized planets, will discover several systems that meet the criteria necessary to determine a star?s weight. Kipping says optimistically: "When they?re found, we?ll be ready to weigh them."

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