Tuesday 27 September 2011

Just how big is Iota Draconis b?

Iota Draconis is a K-type orange giant star in the constellation of Draconis, 103 light years from Earth. It has an apparent magnitude of 3.31, making it naked-eye visible. In 2002 a paper in The Astrophysical Journal by a team lead by Sabine Frink of the Center for Astrophysics and Space Sciences at the University of California, San Diego reported the discovery of a planet orbiting this star, dubbed Iota Draconis b; 'b' implying the second body in the system if it were capitalized, as 'B', it would imply a second star.

The location of Iota Draconis.

The discovery was made at the Lick Observatory at Mount Hamilton in California, where the researchers were studying variations in the radial velocities of K-type stars through minute variations in the Doppler Shift, with a view to discovering binary systems (the radial velocity is movement directly towards or away from the observer). Since the study was looking at a large number of K-type stars it was considered a possibility that large planets might be detected, but this was not the primary purpose of the study; in 2002 exoplanet discovery was a new field and the equipment available at Lick was at the lower resolution limit for this type of detection.

A visualization of how the mass of a planet effects the radial velocity of a star.

Based upon observations and careful modeling of the possible fluctuations in the radial velocity of Iota Draconis, Frink et al. calculated that the star was orbited by a superjovian (bigger than Jupiter) planet once every 536 days. The planet orbited the star at an average distance of 1.3 AU, that is to say 1.3 × the distance at which the Earth orbits the Sun, but with an eccentricity of 0.70. This means that at its closest the planet is only 0.4 AU from the star, and at its furthest 2.2 AU. The mass of the planet could only be calculated by its effect on the star, i.e. by observing the movement of the star, the astronomers were able to calculate the mass of the planet as a ratio of that of the star. At the time the best guess for the mass of Iota Draconis was about 1.05 times that of our sun, giving a minimum mass for the planet of at least 8.9 × that of Jupiter.

This placed Iota Draconis b in a category of planets know as 'Eccentric Jupiters'; planets with masses comparable to or greater than that of Jupiter with highly eccentric orbits. Such planets are very alien to us, as they do not resemble anything in our solar system, but they are one of the best known groups of exoplanets, since their large masses and eccentric orbits make them relatively easy to detect. In the early days of exoplanet-hunting more than 50% of all planets detected were Eccentric Jupiters.

In 2008 a team lead by Mathais Zechmeister of the Max-Planck-Institut für Astronomie published a paper in the journal Astronomy and Astrophysics in which they describe the results of a more long term study of the radial velocity of Iota Draconis from the Lick Observatory and in addition the Thuringia State Observatory in Germany and the McDonald Observatory in Texas, combined with improved methods for calculating the size of K-type giants. Based upon this they calculated that Iota Draconis b orbits its star every 511 days, slightly less than calculated by Frink et al. They also calculated that the star in the system was somewhat larger than originally thought, between 1.4 and 2.2 × the mass of the sun. Based upon this revised mass Iota Draconis b would have a mass at least 10 × that of Jupiter.

On 22 September this year a team lead by Ellyn Baines of the Remote Sensing Division at the Naval Research Laboratory in Washington DC published a paper on the arXiv database at Cornell University Library, in which they detail further study of the Iota Draconis system, using Georgia State University's Centre for High Angular Resolution Astronomy Array interferometer. Using this data they were able to re-calculate the mass of both the star and planet, and the temperature of the star.

Using this new data Baines et al. were able to derive a far more accurate estimate of the mass of Iota Draconis, at 1.82 × the mass of the sun. Using this mass to calculate the size of Iota Draconis b they came up with a mass of 12.6 × that of Jupiter. This is pushing the limits of what can be comfortably described as a planet; some astronomers consider that objects with masses as low as 10 × that of Jupiter should be considered to be brown dwarfs; the International Astronomical Union sets the limit at 13 Jupiter-masses, within the margin of error for the mass of Iota Draconis b. A brown dwarf is considered to be an object to small to fuse hydrogen in its core, but large enough to fuse deuterium (deuterium is an isotope of hydrogen with a proton, giving it an atomic mass of 2, as opposed to 1 for regular hydrogen). In an object with a mass of 13 × Jupiter then this would be a fairly short-lived process; the object would, assuming it could be sampled directly, be defined as a brown dwarf by the absence of deuterium.

Baines et al. were also able to derive an estimate of the surface temperature of 4545K for Iota Draconis (as opposed to 5778K for our sun). From this the derived an estimate for the positioning of a 'Goldilocks Zone' (i.e. a zone in which planets could support life as we understand it) in the Iota Draconis system. This they estimate at occurring between 6.8 and 13.5 AU - far outside the orbit of Iota Draconis b. This is much further out from the star than Earth is, despite the fact that our sun it hotter; this distance is due to the larger size (diameter) of the star. However it seems unlikely that the Iota Draconis system could host inhabitable planets, since they would need to maintain a stable, not-to-eccentric orbit in a system with a very large planet/small brown dwarf in a very eccentric orbit. Furthermore Iota Draconis is a giant star, a star that has completed its main sequence evolution, used up all its hydrogen fuel and switched to the fusion of other heavier elements, causing it to expand. This expansion would have caused the habitable zone to shift outwards within the system, killing any life which did not have the ability to move its planet, though it is in theory possible that life could have started on a planet after the Goldilocks Zone had shifted out to meet it.

An artists impression of the Iota Draconis system.