MIT develops new technique for measuring mass of exoplanets
By David Szondy
December 29, 2013
A team of MIT researchers has described a new method for finding the mass of exoplanets by studying the spectra of light passing through the planet's atmosphere. Because a planet's mass can tell us a lot about its potential for harboring life, this development could provide an important tool in solving the puzzle of whether or not we're alone in the universe.
The frustrating thing about the search for extraterrestrial life is that so many reports make it sound like it's just a matter of finding a planet that's roughly the same size as Earth at a distance from its sun where liquid water could exist, and Bob's your uncle. The trouble is there's a lot more to making a planet habitable than that.
Take the example of our own Solar System. There are three planets that meet the size and distance criteria, but only one has life (no points for guessing which one). The other two, Venus and Mars aren't just near misses. One is like a vision of Hell with sulfuric acid rains and temperatures that could melt lead. The other is so dry that its hard to find a simile and is constantly blasted with radiation. Clearly, there's more to the game than meets the eye.
What life-hunting space scientists need is more data about candidate exoplanets. There are a huge number of factors, but one of the most important is the planet's mass. That's because if you know a planet's size, its mass can tell a lot about its nature. For example, it can tell if the planet in question is a rocky planet, like Earth, or a gaseous one, like Neptune.
Just being able to sift out the Neptune-like planets would speed up the hunt, but knowing the mass of a planet would help scientists deduce other important facts, such as the nature of the atmosphere, whether the planet is tectonically active, and if it has a magnetic field to protect it from cosmic rays and solar winds. All of these, and many other factors, are important in the hunt for life.
The fly in the astronomical ointment is that current techniques for figuring out an exoplanet's mass aren't very useful for studying Earth-like planets. That's because they rely on measuring the wobble of a star due to a planet orbiting about it. This works, but only if the planet is very large or very close to the star, which is exactly what astronomers don't want if they're after Earth-like planets.
The MIT team's solution is to forego wobbles in favor of studying the spectra of light from the exoplanet. It works by waiting until an exoplanet passes in front of its sun, then using a large space telescope, such as NASA's Hubble or Spitzer to record the spectrum of the light as it passes through the planet's atmosphere. Obviously, if a planet doesn't have an atmosphere, it isn't very interesting from a biological point of view.
In addition to allowing astronomers to deduce the planet's size, this This transmission spectrum is used to determine the composition, temperature, and pressure of its atmosphere by means of a standard equation. According to the team, if you have these three, the planet's mass can be calculated.
Lead investigator Juliean de Wit used an 18th-century mathematical constant called the Euler-Mascheroni constant that's used in analysis and number theory and demonstrated that the constant allows one to work out the individual effects of each parameter in the spectrum – described as sort of an “encryption key” to the atmosphere. This was tested using spectra from exoplanet 189733b, located 63 light-years away, and found that the results tracked with the mass figures derived from conventional methods.
“It really helps you unlock everything and reveal, out of these crazy equations, which atmospheric properties do what, and how,” de Wit says. “You find this constant in a lot of physical problems, and it’s fun to see it reappearing in planetary science.”
The MIT team’s results were published in Science.