Gliese 436 b

Gliese 436 b ( /ˈɡliːzə/) is a Neptune-sized extrasolar planet orbiting the red dwarf star Gliese 436.^[6] It was among the smallest known transiting planets in mass and radius until the much smaller Kepler discoveries started coming in 2010.

Discovery

Gliese 436 b was discovered in August 2004 by R. Paul Butler and Geoffrey Marcy of the Carnegie Institute of Washington and University of California, Berkeley, respectively, using the radial velocity method. Together with 55 Cancri e, it was then the first of a new class of planets with a minimum mass (M sini) similar to Neptune.

The planet was recorded to transit its star by an automatic process at NMSU on January 11, 2005, but this event went unheeded at the time.^[7] In 2007, Gillon led a team which observed the transit, grazing the stellar disc relative to Earth. Transit observations led to the determination of Gliese 436 b's exact mass and radius, both of which are very similar to Neptune. Gliese 436 b then became the smallest known transiting extrasolar planet. The planet is about 4000 km larger in diameter than Uranus and 5000 km larger than Neptune and a bit more massive. Gliese 436b (also known as GJ 436b) orbits its star at a distance of 4,000,000 km or 15 times closer than Mercury's average distance from the sun.

Physical characteristics

One orbit around the star takes only about 2 days, 15.5 hours. The planet's surface temperature is estimated from measurements taken as it passes behind the star to be 712 K (439 °C).^[2] This temperature is significantly higher than would be expected if the planet were only heated by radiation from its star (which had been, in a Reuters article from a month prior to this measurement, estimated at 520 K). Whatever energy that tidal effects deliver to the planet does not notably affect its temperature.^[8] Its discoverers allowed for a temperature increase due to a greenhouse effect.^[9]

Its main constituent was initially predicted to be hot "ice" in various exotic high-pressure forms,^[9][10] which remains solid because of the planet's gravity despite the high temperatures.^[11] The planet could have formed further from its current position, as a gas giant, and migrated inwards with the other gas giants. As it arrived in range, the star would have blown off the planet's hydrogen layer via coronal mass ejection.^[12]

However when the radius became better known, ice alone was not enough to account for it. An outer layer of hydrogen and helium up to ten percent in mass would be needed on top of the ice to account for the observed planetary radius.^[2][3] This obviates the need for an ice core. Alternatively, the planet may be a super-earth.^[13]

Observations of the planet's brightness temperature with the Spitzer Space Telescope suggest a possible thermochemical disequilibrium in the atmosphere of this exoplanet. Results published in Nature suggest that Gliese 436b's atmosphere is abundant in CO and deficient in methane (CH₄) by a factor of ~7,000. This result is unexpected because, based on current models at this temperature, the atmospheric carbon should prefer CH₄ over CO.^[14][15]

Orbital characteristics

This planet should not be as eccentric as is measured. To have maintained its eccentricity over time requires that it be accompanied by another planet.^[2][16] In September 2008, a formerly-unrecognised transit signature at NMSU from January 11, 2005 was incorporated into the data up to then, consistent with a planet at 0.08 AU and under 12 Earth masses.^[7]

See also

• HAT-P-11b
• 55 Cancri e
• Gliese 581 b
• Gliese 876 d

References

Selected media articles

• How Do Artists Portray Exoplanets They've Never Seen? 4/9, Scientific American October 2, 2007.
• Astronomers Detect Shadow Of Water World In Front Of Nearby Star (from Science Daily).

External links

Media related to [//commons.wikimedia.org/wiki/Category:Gliese_436_b Gliese 436 b] at Wikimedia Commons

Coordinates: 11^h 42^m 11.0941^s, +26° 42′ 23.652″