Although there have been about 800 extra-solar planets discovered
so far in our galaxy, the precise masses of the majority of them are
still unknown, as the most-common planet-finding technique provides only
a general idea of an object’s mass. Previously, the only way to
determine a planet’s exact mass was if it transits—has an orbit that
periodically eclipses that of its host star. Former Carnegie scientist
Mercedes López-Morales has, for the first time, determined the mass of a
non-transiting planet. The work is published by Astrophysical Journal Letters.

Knowing
a body’s mass is essential first to confirm it is a planet and if so,
to determine whether it is rocky and possibly habitable or large and
gassy. Until now, only the masses of transiting planets have been
measured. Transiting planets are also the only type of extra-solar
objects on which atmospheres have been detected.

López-Morales,
along with her colleagues Florian Rodler and Ignasi Ribas of the
Institute of Space Sciences, ICE (CSIC-IEEC, in Barcelona, Spain)
measured the exact mass of a non-transiting planet. They did this using a
new method that involves studying the carbon monoxide signature of the
planet’s atmosphere—detecting, in the process, the atmosphere of this
non-transiting planet.

The
planet is called Tau Boo b, located in the constellation of Bootes, and
it orbits a star about 50 light years from Earth that’s bright enough
to be visible to the naked eye. The planet is similar in size to Jupiter
and is so close to its star (only 8 stellar radii), that a year for
this planet asts only 3.3 Earth days. Furthermore, its surface
temperature reaches 1,500 C, making it inhospitable to life.

Discovered
in 1996, Tau Boo b was one of the first planets originally detected by
the radial velocity method. This planet does not transit, but its
presence and characteristics were initially determined by the wobble of
its host star. This technique only provides a rough indication of a
detected planet’s mass.

In
June 2011, López-Morales’ team conducted five hours of observations at
near infrared wavelength (2.3 microns). They obtained data from the
high-resolution spectrograph CRIRES, an instrument mounted on one of the
four 8.2m Very Large Telescopes (VLT) of the European Southern
Observatory (ESO) in Chile.

The
observations and subsequent data analysis revealed the presence of
carbon monoxide in the planet’s atmosphere. In addition, by studying the
planet’s orbital motion through the displacement of spectral lines of
carbon monoxide, the team was able to calculate its exact mass—5.6 times
Jupiter—a first using this particular method, and also a first for a
non-transiting planet.

An
independent study conducted by researchers at the University of Leiden
in the Netherlands obtained a similar result for the same planetary
system, confirming the potential of this technique.

“This
method represents a strong advance in the field of exoplanets,” said
Lopez-Morales. “It opens a new path to determine masses of exoplanets
and the composition of their atmospheres”

The
research team expects many more planets will be weighted using this new
technique. They are also convinced that in the future, they will be
able to detect molecules that are associated with the presence of life
in non-transiting distant planets.”

Source: Carnegie Institution