Friday, 17 January 2014

Three giant exoplanets in very wide orbits around young stars.

In the past two decades a large number of planets have been discovered orbiting other stars (exoplanets). The vast majority of these have been large planets orbiting close to their host stars, such planets being easier to detect due to the influence that their gravity has on the star. Planets further from their stars are harder to detect, as their gravity has less effect upon the star, and they have long orbital periods which will tend to mask this anyway. Such planets are more likely to be detected by direct imaging, though this will require separate observations over a long period of time to confirm the relationship with the host star.

In a paper published on the online arXiv database at Cornell University Library on 29 November 2013 and accepted for publication in The Astrophysics Journal, a team of scientists led by Adam Kraus of the Department of Astronomy at the The University of Texas at Austin and the Harvard-Smithsonian Center for Astrophysics describe three giant planets orbiting young stars at distances in excess of 100 AU (i.e. more than 100 times the distance at which the planet Earth orbits the Sun). All three bodies had previously been noted as potential planets over a decade ago, but can only now be confirmed as objects in orbit about their stars, due to follow-up observations that have tracked their movement. 

Large planets in very wide orbits about young stars presents a considerable challenge for conventional models of planet formation, since it should in theory take far longer for a planet to form this far from a star, potentially never accreting into a large body at all. Our own system contains considerable material beyond the orbit of Neptune (30 AU from the Sun), but this has apparently never accreted into a large planet, despite the 5 billion year age of the Solar System.

The first of the three new planets orbits the binary system FW Tau AB near the center of the Taurus- Auriga complex, 473 light years from Earth. The FW Tau system comprises two red dwarf stars  (FW Tau A and FW Tau B) each though to have a mass 28% of our Sun's orbiting one another at a distance of 11 AU (11 times the distance between the Sun and the Earth). The system is thought to be about 1.8 million years old. The planet, FW Tau b (when naming objects in other stellar systems stars are given upper case letters and planets lower case letters), is calculate to have a mass 10 times that of Jupiter and orbit at a distance of 330 AU (over 10 times the distance at which Neptune orbits the Sun). 

Infrared image of the FW Tau system. The image is not coronagraphic; most of the image is shown with a linear stretch that saturates at 110% of the peak brightness of the wide companion, while a box of size 0.5′′ is instead shown with a linear stretch that saturates at 110% of the peak brightness of the primary star in the close binary. North is up. The scale bar is 1 arc inch; i.e. 1/21 600th of the circumference of an imaginary sphere around the Earth. Kraus et al. (2013).

The second new planet orbits the binary star ROXs 42B, 440 light years from Earth in the constellation of Ophiuchus. The binary system comprises two stars with masses of 89% that of the Sun and 36% that of the Sun, orbiting at a distance of less than 10 AU. The system is thought to be 6.8 million years old. The new planet, ROXs 42B b is calculated to orbit this pair at a distance of 140 AU and have a mass 10 times that of Jupiter. A second, potential body in the system, provisionally dubbed ROXs 42B cc1, is thought to be a background star.

Infrared image of the ROXs 42B system. The image is not coronagraphic; most of the image is shown with a linear stretch that saturates at 110% of the peak brightness of the wide companion, while a box of size 0.5′′ is instead shown with a linear stretch that saturates at 110% of the peak brightness of the primary star in the close binary. North is up. The scale bar is 1 arc inch; i.e. 1/21 600th of the circumference of an imaginary sphere around the Earth. Kraus et al. (2013).

The third new planet orbits ROXs 12, another star in the constellation of Ophiuchus, 391 light years from Earth. ROXs 12 is thought to have a mass 87% of that of the Sun and to be 7.6 million years old. ROXs 12b orbits this star at a distance of 210 AU, and has a mass 16 times that of Jupiter.

Infrared image of the ROXs 12 system. The image is not coronagraphic; most of the image is shown with a linear stretch that saturates at 110% of the peak brightness of the wide companion, while a box of size 0.5′′ is instead shown with a linear stretch that saturates at 110% of the peak brightness of the primary star in the close binary. North is up. The scale bar is 1 arc inch; i.e. 1/21 600th of the circumference of an imaginary sphere around the Earth. Kraus et al. (2013).


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