Gliese 581 / HO Librae

As of April 21, 2009, Gliese 581 is believed to host at least four planets between the size of Earth and Neptune, with one orbiting the star roughly within its habitable zone. On November 30, 2005, a team of French and Swiss astronomers announced the discovery of a Neptune-class planet in a very close "torch" orbit around Gliese 581 (ESO press release and Bonfils et al, 2005, in pdf -- details below). On April 25, 2007, astronomers using the European Southern Observatory's 3.6-meter telescope announced the discovery of two additional lower-mass planets in this system, including the most Earth-like planet found outside the Solar System to date (a planet with a diameter that is one and a half as much wider than Earth with at least five times Earth's mass of the Earth) and another planet with at least eight Earth-masses, one of which may be capable of having liquid water on its surface (ESO news release; Udry et al, 2007; von Bloh et al, 2007; Observatoire de Genève's web page on Gl 581; and Astronomy Picture of the Day -- more below). On April 21, 2009, a team of astronomers at the 2009 European Week of Astronomy and Space Science conference announced the discovery of another planetary candidate "e" with a minimum mass of 1.9 Earth-masses in an innermost orbit and revised orbital analysis indicating that planetary candidate "d" appears to be located in the so-called habitable zone of Gliese 581 (ESO press release; and Mayor et al, 2009 -- 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.)

© ESO




Larger illustration.




Gliese 581 appears
to host at least
four planets between
Earth and Neptune
in mass (more).

This star was probably first designated as BD-07 4003 and added to a catalogue that was originally published in 1863 by Friedrich Wilhelm August Argelander (1799-1875) on the position and brightness of 324,198 stars between +90° and -2° declination that were measured over 11 years from Bonn, Germany with his assistants Eduard Schönfeld (1828-1891) and Aldalbert Krüger (1832-1896). This catalogue became famous as the Bonner Durchmusterung ("Bonn Survey") and is typically abbreviated as BD. It was later expanded and extended during the early 20th Century with the Cordoba (observed from Argentina) then the Cape Photographic Durchmusterung (observed from South Africa).

Its high proper motion was probably discovered photographically, and it was designated as Wolf 562 by Max (Maximilian Franz Joseph Cornelius) Wolf (1863-1932). Wolf was a pioneer of astrophotography who discovered hundreds of variable stars and asteroids, and about 5,000 nebulae by analyzing photographic plates and developing the "dry plate" in 1880 and the "blink comparator" in 1900 with the Carl Zeiss optics company in Jena, Germany. Today, many astronomers refer to this star by its earliest designation as Gl 581 in the first 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).

The Star

Gliese 581 is a cool and dim, main sequence red dwarf (M2.5 V). The star has almost a third (31 +/- 2 percent) of Sol's mass, possibly 38 percent of its diameter (Pasinetti-Fracassini et al, 2001; and Johnson and Wright, 1983), and a bit more than one percent (around 0.013) of its bolometric luminosity (Bonfils et al, 2005, in pdf). The star appears to be between 36 to 62 percent as enriched as Sol in elements heavier than hydrogen (Bean et al, 2006; Ken Croswell, 2006; and Bonfils, et al, 2005). Its kinematic characteristics, magnetic activity, and sub-Solar metallicity indicate that that Gliese 581 is at least two billion years old, possibly around 4.3 billion years old (Udry et al, 2007). Moreover, the Canadian Space Agency's MOST satellite recently monitored Gliese 581 for a month and a half and found that the brightness of the star varied by only a few tenths of a percent, which suggests that the star is probably several billions years old (more). Gliese 851 has been previously classified as a variable star with the designation "HO Librae" and NSV 7023. Some other useful star catalogue designations include: HO Lib, Gl 581, Hip 74995, BD-07 4003, G 151-46, G 152-9, LHS 394, LTT 6112, LPM 564, LFT 1195, Vys/MCC 159, and Wolf 562.

© ESO





Larger illustration.





Gliese 581 appears
to host at least
four planets, one
within its habitable
zone (more).

Planetary System

As of April 21, 2009, Gliese 581 is believed to host at least four planets between the size of Earth and Neptune, with one orbiting the star roughly within its habitable zone.

On November 30, 2005, a team of French and Swiss astronomers announced the discovery of a Neptune-class planet in a very close "torch" orbit around this star using the HARPS spectrograph on the European Southern Observatory's 3.6-meter telescope at La Silla, Chile (ESO press release) and Bonfils et al, 2005, in pdf). Initially estimated with at least 16.6 Earth-masses, the mass of Planet "b" has been more recently updated to at least 15.65 times (maximum of 30.4) that of the Earth. With a semi-major axis of 0.04 AUs (around 6 million kilometers), it orbits so close to its host star that its orbital period lasts only 5.36874 +/- 0.00019 days, in a highly circular orbit (e~0) (Mayor et al, 2009; and Udry et al, 2007). As a result, the planet must be very hot, about 147 degrees Celsius (420° Kelvin).

JPL, NASA



Larger image.



It is not yet known whether
planetary candidate "b" is
rocky or gaseous -- like
Neptune.

On April 25, 2007, a team of Swiss, French and Portuguese astronomers (again using the HARPS spectrograph on the European Southern Observatory's 3.6-meter telescope) announced the discovery of two additional lower-mass planets in this system, including the most Earth-like planet found outside the Solar System to date (ESO news release; Udry et al, 2007; Observatoire de Genève's web page on Gl 581; and Astronomy Picture of the Day). The more Earth-like planet "c" has at least 5.36 (maximum of 10.4) Earth-masses and an estimated diameter of at least 1.5 times Earth's if it is rocky, larger if it is composed of mostly ice. It moves around Gliese 581 -- outside of planet b's orbit -- at an average distance of 0.07 AU, in a circular orbit (e=0.17 ± 0.07) which it completes in 12.9292 +/- 0.0047 days Mayor et al, 2009). Initially thought to be located within the so-called habitable zone of Gliese 581 (which is now thought to be between around 0.11 and 2.8 AUs from the star), the planet may have a mean temperature between -3 and over 40 degrees Celsius (27 and 104+ degrees Fahrenheit) with water on its surface, depending on whether the planet has a Venus- or an Earth-like albedo (Udry et al, 2007), although subsequent modelling suggests the planet is likely to be to close to the star resulting in a Venus-like runaway greenhouse effect which would boil off all surface water (von Bloh et al, 2007). (Illustrations of one artist's vision of tidally locked day- and night-side surfaces of planet c are available from Karen Wehrstein and Astronomy Picture of the Day.)

© ESO




Larger illustration.




Planet "c" may be able
to have liquid water on
its surface under the
right atmosphere, among
other necessary planetary
conditions (more).