Gliese 436 / AC+27 28217

On August 31, 2004, astronomers announced the discovery of a Neptune-sized planet in a scorchingly hot inner orbit around Gliese 436 (more below -- NASA press release and NSF press release). On May 16, 2007, a team of astronomers announced that measurements of the planet's width suggest that it may be made mostly of two exotic forms of hot, pressurized ice (Gillon et al, 2007 -- more below). On January 21, 2008, a team of astronomers released a draft paper on the possible discovery of a planet with only around five Earth-masses just outside the orbit of the previously discovered Neptune-sized planet around Gl 436 (Ribas et al, 2008 -- more below). On September 13, 2010, one team of astronomers announced that the atmosphere of Neptune-size planet "b" has 7,000 times less methane (CH₄) than predicted by models, but another team of astronomers believes that the finding is due to observational or analytical flaws and that the issue can be resolved with the collection of additional spectroscopic data (Science@NASA; Stevenson et al, 2010; and Beaulieu et al, 2010 -- more below). (See an animation of the planetary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.)

Unknown artist,
JPL, CalTech, NASA



Larger and jumbo images.



Hot, inner planet "b" may have
less atmospheric methane than
models predict (more).

Many astronomers now refer to this star as Gliese 436 by its designation in the famous Gliese Catalogue of Nearby Stars (CNS, now ARICNS database) of Wilhelm Gliese (1915-93), who was a longtime astronomer at the Astronomiches Rechen-Institut at Heidelberg (even when it was at Berlin). However, the star may have been designated first as AC+27 28217 in the Catalogue Astrographique (Astrographic Catalogue), part of the international "Carte du ciel" program. The program was designed to photograph and measure the positions of all stars brighter than magnitude 11.0, but over 4.6 million stars eventually were observed, including many as faint as 13th magnitude. From 1891 to 1950 (but mostly between 1895 and 1920), 20 observatories exposed and measured the plates of their zones of the sky, using a standardized telescope so each plate photographed had a similar scale of approximately 60 arcsecs/mm (more). The U.S. Naval Observatory has taken over custody of the catalogue and issued the AC 2000.2 edition.

The Star

Gliese 436 is a relatively dim, main sequence orange-red or red dwarf star of spectral and luminosity M2.5-3.5 V (Butler et al, accepted Dec. 2004, in pdf; and ARICNS). The star may have 44 to 51 percent of Sol's mass (von Braun et al, 2012; Maness et al, 2007; and Butler et al, accepted Dec. 2004, in pdf), around 42 to 46 percent of its diameter (von Braun et al, 2012; and Johnson and Wright, 1983, page 673), and 2.5 percent of its bolometric-corrected luminosity. Based on kinematic and chromospheric analysis, it may be over 11 billion years old and a member of the Milky Way's old disk stellar population (Saffe et al, 2005). Gliese 436 is only around 48 percent as enriched as Sol in elements heavier than hydrogen (Bean et al, 2006; and Ken Croswell, 2006). Some other useful star catalogue designations include: AC+27 28217, Gl 436, Hip 57087, LHS 310, LTT 13213, LFT 838, G 120-68, G 121-7, G 147-68, Vys 616, and Ross 905.

Planetary System

JPL, NASA



Larger image.



Hot, inner planetary candidate "b" orbits
Gliese 436 within 0.028 AUs (more
images and animations at Planet Quest).

Planet "b" - On August 31, 2004, astronomers (R. Paul Butler, Steve S. Vogt, Debra A. Fischer, Geoffrey W. Marcy and Jason T. Wright, Gregory W. Henry, Greg Laughlin, and Jack J. Lissauer) announced the discovery of a Neptune-sized inner planet (NASA/JPL press release; NSF press release); Maness et al, 2007; and Butler et al, accepted Dec. 2004, in pdf). With around 24.8 +/- 2.2/5 Earth-masses (Gillon et al, 2007), or 0.078 ± 0.007/8 Jupiter-masses (von Braun et al, 2012; and Guillermo Torres, 2007) and 4.1 times Earth's diameter (von Braun et al, 2012), planetary candidate "b" orbits Gliese 436 of less than three days -- 2.64385 (± 9e-05) days (Maness et al, 2007). Its scorchingly close, relatively circular inner orbit has a semi-major axis of 0.02872 (± 0.00027) AUs or approximately 4.1 million kilometers or 2.6 million miles (Guillermo Torres, 2007; and Maness et al, 2007) -- less than a tenth of Mercury's orbital distance in the Solar System. Planet b's orbit has an eccentricity of about e= 0.137 with an orbital inclination of around 86.7 degrees (Stevenson et al, 2012, Table 3; and Deming et al, 2007; Maness et al, 2007; and Gillon et al, 2007), like most giant planets found orbiting close to their parent stars, which suggests that there may be another large planet in an outer orbit. Initially, astronomers did not have enough planetary data to determine if this planet is gaseous or rocky like Earth and Mars, but one estimate of the planet's temperature suggests around 370° C (698° F) on the daylight side, but possibly colder than 200° C (392° F) on a possibly, tidally locked dark side.

JPL, NASA



Larger image.



Initially, astronomers were not sure
whether planet "b" is mostly rocky
a hot "ice giant" (like Neptune) at
around 1,500° C (or 2,700° F).

On May 16, 2007, a team of astronomers (including Michael Gillon, Frédéric Pont, B.-O. Demory, Frédéric Mallmann, Michel Mayor, Didier Queloz, Avi Shporer, Stephane Udry, and Christel Vuissoz) announced that measurements of the planet's width at 3.95 +/- 0.35 Earth's diameter, close to that or Uranus and Neptune in the Solar System. They deduce that a planet of such size and mass located to its host star may be composed mostly of two exotic forms of hot, pressurized ice (Ice VII and X), as an "ice giant" type of planet more like Uranus and Neptune rather than a predominantly rock/iron "super-Earth" or a low-mass giant (press release and new illustrations at U.C. Berkeley and exoplanets.org). The surface may be very hot, exceeding 300° C (570° F), keeping water in its atmosphere in vapor form. In addition, an outer layer of hydrogen and helium gas totalling up to 10 percent the planet's mass is probably needed on top of the ice to account for the observed planetary width (Gillon et al, 2007). According to planetary scientist and physicist Sara Seager, however, other planetary structures and compositions (including a rocky world with a massive atmosphere) may still fit the available data. If the ice composition hypothesis is correct, however, then the planet may have formed further out from the star (where it was sufficiently cool for water and/or ice to condense out out of the circumstellar dust disk) but migrated into its present inner orbit.

Frédéric Pont,
Observatoire de Genève


Larger image.


Based on its width,
mass, and proximity
from Gl 436, planet b
is now thought by
some astronomers
to be made mostly
of hot, pressurized
water ice in exotic
forms (Ice VII and X).