# How to weigh a star using a moon

##### October 15, 2010

How do astronomers weigh a star that's trillions of miles away and way too big to fit on a bathroom scale? In most cases they can't, although they can get a best estimate using computer models of stellar structure.

New work by astrophysicist David Kipping says that in special cases, we can weigh a star directly. If the star has a planet, and that planet has a , and both of them cross in front of their star, then we can measure their sizes and orbits to learn about the star.

"I often get asked how astronomers weigh . 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.

Astronomers have found more than 90 that cross in front of, or transit, their stars. 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 unless they know the actual size of the star. Computer models give a very good estimate but in science, real measurements are best.

Kipping realized that if a transiting planet has a moon big enough for us to see (by also blocking starlight), then the planet-moon-star system could be measured in a way that lets us calculate exactly how large and massive all three bodies are.

"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.

The process isn't easy and requires several steps. By measuring how the star's light dims when planet and moon transit, astronomers learn three key numbers: 1) the orbital periods of the moon and planet, 2) the size of their orbits relative to the star, and 3) 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 gives 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.

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

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.

"When they're found, we'll be ready to weigh them," said Kipping.

Explore further: Wobbly planets could reveal Earth-like moons

## Related Stories

#### Wobbly planets could reveal Earth-like moons

December 11, 2008

Moons outside our Solar System with the potential to support life have just become much easier to detect, thanks to research by an astronomer at University College London (UCL).

#### Detecting Life-Friendly Moons

October 26, 2009

The search for life-friendly real estate around distant stars doesn't have to be limited to planets. New research shows that habitable exomoons can be detected with a new method using current technology.

#### Transit Search Finds Super-Neptune

January 20, 2009

(PhysOrg.com) -- Astronomers at the Harvard-Smithsonian Center for Astrophysics have discovered a planet somewhat larger and more massive than Neptune orbiting a star 120 light-years from Earth. While Neptune has a diameter ...

#### Will Kepler find habitable moons?

September 3, 2009

(PhysOrg.com) -- Since the launch of the NASA Kepler Mission earlier this year, astronomers have been keenly awaiting the first detection of an Earth-like planet around another star. Now, in an echo of science fiction movies ...

#### Avatar's moon Pandora could be real

December 17, 2009

In the new blockbuster Avatar, humans visit the habitable - and inhabited - alien moon called Pandora. Life-bearing moons like Pandora or the Star Wars forest moon of Endor are a staple of science fiction. With NASA's Kepler ...

#### Kepler mission discovers two planets transiting same star

August 26, 2010

(PhysOrg.com) -- NASA's Kepler spacecraft has discovered the first confirmed planetary system with more than one planet crossing in front of, or transiting, the same star.

## Recommended for you

#### Giant radio flare of Cygnus X-3 detected by astronomers

December 7, 2016

(Phys.org)â€”Russian astronomers have recently observed a giant radio flare from a strong X-ray binary source known as Cygnus X-3 (Cyg X-3 for short). The flare occurred after more than five years of quiescence of this source. ...

#### Dark matter may be smoother than expected

December 7, 2016

Analysis of a giant new galaxy survey, made with ESO's VLT Survey Telescope in Chile, suggests that dark matter may be less dense and more smoothly distributed throughout space than previously thought. An international team ...

#### To Mars in 70 days: Expert discusses NASA's study of paradoxical EM propulsion drive

December 7, 2016

After months of speculation and rumor, NASA has finally released its long-awaited research paper on the controversial EM Drive propulsion system. The paper was recently published in the American Institute of Aeronautics and ...

#### Cassini transmits first images from new orbit

December 7, 2016

NASA's Cassini spacecraft has sent to Earth its first views of Saturn's atmosphere since beginning the latest phase of its mission. The new images show scenes from high above Saturn's northern hemisphere, including the planet's ...

#### New evidence for a warmer and wetter early Mars

December 7, 2016

A recent study from ESA's Mars Express and NASA's Mars Reconnaissance Orbiter (MRO) provides new evidence for a warm young Mars that hosted water across a geologically long timescale, rather than in short episodic bursts ...

#### ExoMars orbiter images Phobos

December 7, 2016

The ExoMars Trace Gas Orbiter has imaged the martian moon Phobos as part of a second set of test science measurements made since it arrived at the Red Planet on 19 October.

##### NotAsleep
not rated yet Oct 15, 2010
How have we determined the masses of stars in the past? And what would it mean to our current theories if Kipping's method produced significantly different results than what we already assume is correct?
##### PinkElephant
5 / 5 (7) Oct 15, 2010
How have we determined the masses of stars in the past?
Mostly, by measuring the spectrum of the star's light, as well as its brightness and distance, and then comparing against the established "stellar sequence" theory:

http://en.wikiped...sequence

Additionally, in the case of Type 1A supernovae, their masses are pretty well-constrained due to the unique mechanism driving the supernova.
if Kipping's method produced significantly different results than what we already assume is correct?
That would mean we're missing some major physics in regard to the structure and internal dynamics of stars.