This week’s cosmic quiz question was submitted by Daniel Wallace, who asked, “How do astronomers figure out the size of planets orbiting other stars (and even what their atmosphere is made of)?”
To date astronomers have discovered well over three hundred planet orbiting stars other than our own Sun, and that number is growing rapidly. We call these planets orbiting other stars “exoplanets.”
Think about that for a minute… Today we are aware of forty times more planets outside of our solar system than we know of inside our solar system.
There are several methods used to discover exoplanets, but one of the most commonly used methods involves looking at how the gravitational attraction between the exoplanet and its central star causes the star to wobble as the exoplanet orbits.
Even though most stars are many thousands of times more massive than any planet that orbits them, a really massive planet, such as Jupiter (which is more than 300x the mass of Earth), can exert enough of a gravitational tug on a smallish star to cause the star to shift back and forth a tiny but measurable amount as the planet moves around the star.
Starlight can tell you a lot about a star. By carefully studying a star’s light astronomers can determine the star’s chemical makeup, temperature, mass, size and age. By combining the information gained from the star’s spectrum with a few other clever observations astronomers can also determine the star’s distance from Earth and its velocity through space.
All that information can be found by analyzing the light coming from a star. As an example, here’s an image of our Sun’s spectrum:
All those little dark lines spread out over the different colors of sunlight represent chemical elements that are present in the Sun’s atmosphere.
If a star is approaching us the lines are shifted slightly towards the blue end of the spectrum. If the star is receding from us the lines are shifted slightly towards the red end of the spectrum.
Gravity works in both directions between a star and any exoplanet in orbit around it. The star tugs on the exoplanet, and the planet tugs back on the star. When astronomers find a star that periodically appears to be approaching and then receding from Earth they know they’re looking at a star that’s doing a cosmic do-si-do with one or more exoplanets in orbit around it.
The magnitude, duration, and complexity of the periodic changes in the star’s spectrum tells astronomers the mass, distance and period of orbit of the exoplanet.
By knowing the exoplanet’s mass and distance from its star, and the temperature, mass and brightness of the central star itself, astronomers can estimate the exoplanet’s size, composition and surface temperature.
Another way to discover planets is to observe the slight dimming of a star’s light when an exoplanet happens to travel directly in front of the star as seen from Earth.
Even though this is roughly the equivalent of detecting changes in a car headlight’s brightness from a mile away as a gnat crawls across the headlight, astronomers have instruments that are indeed sensitive enough to make this kind of measurement.
This is a very exciting time for the study of exoplanets. NASA has recently launched the Kepler Mission, an orbiting spacecraft that will study stars to look for the passage of Earth-sized planets in front of their primary stars.
Many exoplanets have been discovered using this technique, but one at a time. Kepler is going to look at more than 100,000 stars simultaneously.
Even more exciting, as the exoplanet passes in front of its primary star, the light of the star passes through any atmosphere the exoplanet might have, and carries the spectral chemical signatures of the atmosphere along with the rest of the star’s spectrum to Earth. By subtracting the spectrum of the star plus planet from the spectrum of the star alone, it’s possible to study the chemistry of the exoplanet’s atmosphere.