Q: How do scientists know what planets are made of? (Harsja, Ubud, Bali)

A planet found recently, 500 light years away. A team led by Timothy Brown (NCAR), David Charbonneau (CfA), and Edward Dunham (Lowell Observatory) led the discovery effort.  Drawing courtesy of David A. Aguilar, Harvard-Smithsonian Center for Astrophysics.

A:  Scientists determine a planet's ingredients in several ways, depending partly on how close to Earth the planet is. 

Planets within our solar system:  For planets like Mars that robots have visited, we analyzed rock and atmospheric samples on the spot.  We also measure the planet's magnetic properties. 

To detect a magnetic field, the planet must be close enough for a flyby.  When the spacecraft nears the planet, we measure "how the star's stellar wind is deflected," says professor Nick Strobel of Bakersfield College, or use an instrument called a magnetometer.

Planets outside the solar system:  For planets too distant to fly by or go to, we rely on an analysis of light received from the planet and the laws of physics.  We crack that distant-planet puzzle in a two-step procedure: 

• learning a planet's mass and volume gives us its average density, which reveals something about its ingredients.
• analyzing light from the planet tells us about its atmosphere.

Density:  We determine a planet's makeup primarily by learning its density.  For example, Earth's density is about 5.5 times water's density.  So, if we learn that a new planet has about the same density as Earth's, we might guess it is similar to Earth.  Its ingredients are probably silicate rock surrounding a heavier iron-nickel core. 

On the other hand, a density in the range of 0.7 to 1.7 indicates the planet is more like Jupiter or Saturn:  a gas world with thick atmosphere.

It takes detective work, though, to make educated guesses about density.  We must first learn the volume and the mass of the unknown body, since density is the mass divided by the volume.   

The distant star moves towards Earth (left image) and, like an approaching train's steam whistle rises in pitch, its light shifts to the blue. Drawing courtesy of Gilbert Esquerdo of the Planetary Science Institute.

Mass:   To determine mass, "we use the Doppler method to detect the slight wobble in velocity of the parent star due to the planet's tugging it back and forth.  The larger the planet's mass, the more it tugs the star, and the greater the change in the star's velocity," emails astronomer David Charbonneau of the Harvard-Smithsonian Center for Astrophysics. 

The star and planet spin about their common center of gravity — balanced like two (extremely unequal) weights  on a teeter-totter, says Esquerdo.  See figure.  As the planet moves away from their center of gravity (the left side of the figure), the star moves in the opposite direction, towards Earth, and the star's light shifts to the blue spectra.  When the planet moves toward the center of gravity (and Earth, right image), the star moves away from Earth and its light shifts to the red, like a receding train's whistle drops in pitch.

The accuracy of this measurement has increased by a thousand-fold in the past 50 years.  We can now measure small enough velocity changes (a few m/s, less than 6 mph) to detect the effects on its star of a Earth-like planet the size of Neptune.

Volume:  To get the planet's diameter, we use eclipse information, as we did with Pluto and Charon.  We measured how the light intensity diminished when Charon passed in front of Pluto and when Pluto passed in front of Charon. 

Indeed, "The only technique by which we have ever determined the size of planets orbiting other stars is by observing the dip in the light intensity as the planet occults a portion of the stellar surface," says Charbonneau. 

Knowing the diameter, we assume the body is a sphere to calculate its volume.

Atmosphere:  To learn a planet's atmosphere, we examine the spectrum of starlight passing through its atmosphere.  Missing frequencies are clues, indicating elements or compounds that absorb light at those frequencies are present in the atmosphere.  For example, if the light frequencies corresponding to methane and carbon monoxide are missing from an analysis of the starlight, the atmosphere contains methane and carbon monoxide, which absorbed the missing light.

Further Reading:

Periodic dimming of bright starlight reveals distant planet, Astrophysical Institute of the Canaries, National Center for Atmospheric Research, Harvard Smithsonian Center for Astrophysics, Lowell Observatory and California Institute of Technology.

Detecting planets by Nick Strobel, Bakersfield College

Detecting a planet's magnetic field, Windows to the Universe