Calculating the mass of a non-transiting exoplanet

Cosmologists have been scouring the skies in an effort to find another Earth-like planet. While hundreds of exoplanets have been found over the past couple of years, it's not always easy to determine which, if any, could support life. To do so, we have to know the composition (including any atmosphere), mass and orbital distance of each prospect.

Not all planet-finding techniques are equal when it comes to figuring out these parameters. Physicists can calculate the mass of exoplanets found using the transit method (detecting the change in light coming from a star as a planet passes in front of it). However, not all planets pass in front of their stars from our vantage point. A variety of other methods have been used to find these planets, including the radial velocity method, which detects planets by the slight gravitational wobble they give their parent stars. Until recently, cosmologists have only been able to provide the lower limit of masses for planets that do not transit their stars. Now, however, two papers have come out describing how the true mass of the non-transiting exoplanet Tau Boötis b (also called Tau Boo b) was determined.

Mercedes López-Morales from the Carnegie Institution of Washington and Florian Rodler and Ignasi Ribas of the Institute of Space Sciences in Barcelona, the University of Toronto and MIT found evidence of carbon monoxide (CO) in Tau Boötis b’s atmosphere. They were able to use the shifting of the spectral absorption data of the CO over time to calculate that the planet orbits its star at a 44^o angle. This in turn allowed the astronomers to calculate the mass of Tau Boötis b.

Matteo Brogi and his colleagues from Leiden University in the Netherlands used a similar method to determine the mass of Tau Boötis b. By careful scrutiny, the astronomers were able to directly observe the orbiting planet.  They too came up with the same 44^o inclination and mass.

Tau Boötis b orbits the star Tau Boötis, which is about fifty light years away and visible with the naked eye. Despite being six times the mass of Jupiter, Tau Boötis b orbits its star far closer than Mercury does our sun. This gives it a surface temperature above 1800 K. In short, Tau Boötis b is not accommodating to life.  Nevertheless, this method could be used to determine the masses of other exoplanets that might be more hospitable.

Below is an artist's rendition of the planet. You can also see some animations of the system here.

Credit: ESO/L Calcada.