Friday, 18 May 2012

Thermal imaging 55 Cancri e.

55 Cancri (or Rho¹ Cancri) is a binary star system in the constellation of Cancer, 40 light years from Earth; it is naked eye visible. It comprises two stars, 55 Cancri A, a Sun-like G-type star with 95% of the Sun's mass, 1.15 times its radius (6.4 times its volume) and an effective temperature of 5373 K (compared to 5778 K for our Sun), which is orbited by 55 Cancri B, a smaller, cooler Red Dwarf star, at a distance of 1000 AU (i.e 1000 times the distance at which the Earth orbits the Sun, or 33.3 times the distance at which Neptune orbits). The system also has at least 5 planets.

Finding 55 Cancri within the constellation Cancer. University of Illinois.

The first of these, 55 Cancri b (or 55 Cancri Ab to distinguish it from 55 Cancri B) was detected in April 1996, by scientists at the Lick Observatory in California, who were able to detect the wobble of 55 Cancri A caused by the gravity of the planet as it orbits the star. 55 Cancri b orbits 55 Cancri A at a distance of 0.115 AU (11.6% of the distance at which the Earth orbits the Sun, less than half the orbital distance of Mercury) every 14.6 days. It is thought to have a mass slightly over that of Jupiter.

Two further planets were discovered in June 2002, by the same team, using a refinement of the technique that enabled subtler influences on the star's movement to be detected. 55 Cancri c has a mass 0.169 times that of Jupiter (54 times that of Earth), and orbits 55 Cancri A every 44.3 days at a distance of 0.24 AU (two thirds of Mercury's orbit). 55 Cancri d has a mass 3.8 times that of Jupiter and orbits 55 Cancri A at a distance of 5.77 AU (comparable to 5.2 AU for Jupiter) every 5218 days (14.3 Earth years).

In August 2004 a fourth planet was discovered, using the Hobby-Eberly Telescope in Texas, again by its gravitational influence on the star. 55 Cancri e has a mass 7.81 times that of Earth and orbits 55 Cancri A at a distance of 0.0156 AU, every 18 hours.

In April 2005 a fifth planet, 55 Cancri f, was discovered using further refinements on the gravitational wobble of 55 Cancri A, and data from the Lick and Keck Observatories. This planet orbits at 0.78 AU every 260 days, placing it firmly within 55 Cancri's habitable zone (the zone in which a reasonably Earthlike planet might host liquid water, and therefore potentially life), though with a mass of at least 0.144 that of Jupiter (at least half that of Saturn) 55 Cancri f is unlikely to do so.

Diagram showing the relative orbits of the known planets of 55 Cancri. Orbits of the planets of our Solar system (grey) shown for comparison. The Visual Exoplanet Catalogue.

Uniquely among these planets, 55 Cancri e transits (passes in front of) its star, when seen from Earth, giving it greater potential for further study. In a paper published on the arXiv database at Cornell University Library on 8 May 2012, and accepted for publication in The Astrophysical Journal Letters, a team of scientists led by Brice-Oliver Demory of the Department of Earth, Atmospheric and Planetary Sciences at the Massachusetts Institute of Technology preset the result of a study of 55 Cancri e's infrared emissions during a series of transits of 55 Cancri A, made using the Spitzer Space Telescope.

They obtained an estimated temperature of 2360 K for 55 Cancri e, which is hot, but cooler than expected for a planet this close to a star. This suggests it has a low Bond albedo; i.e. it reflects less of the infrared radiation that falls onto it back into space than would be expected. This could be caused by a temperature inversion in the atmosphere, i.e. a warmer layer of gasses trapped below a cooler one, but this would need to be very large (over 500 K) which is improbable. Alternatively it could imply that the planet is rocky with little atmosphere (Mercury and the Moon both have low Bond albedos), which would be surprising for a planet of this mass, but could be explained by its proximity to 55 Cancri A, which could have evaporated its atmosphere away (the 55 Cancri system is 7.4-8.7 billion years old).

An artist's impression of 55 Cancri e. NASA/JPL.


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1 comment:

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