Gliese 581 / HO Librae

On July 18, 2012, a team of astronomers released a paper that revealed new data supporting the existence of planetary candidate "g," previously announced in 2010. Their new analysis of an extended dataset of observations also strengthens their original assumption that all the previously discovered planets around Gliese 581 are in circular rather than elliptical orbits as currently believed. It is under this likely assumption that the Gliese 581g signal appears in the new data. With a "False Alarm Probability of < 4%," the results of the new analysis is consistent with a planet with at least 2.2 Earth-masses in a habitable zone orbit at 0.13 AU from the host red dwarf star, "where liquid water on planetary surfaces is a distinct possibility.” (PHL press release; and Vogt et al, 2012).

PHL; Vogt et al, 2012



Larger and jumbo illustrations.




Analysis of an expanded dataset
of observations supports the
presence of Gliese 581 g in a
circular, habitable zone orbit
among its sibling planets (more).

On May 5, 2011, a team of scientists submitted a pre-print on the results of climate model simulations which further clarified under what conditions that planet d might be warm enough to support liquid water on its surface. At its current orbital distance from its dim red dwarf, host star, planet d receives "35% less stellar energy than Mars and is probably locked in tidal resonance, with extremely low insolation at the poles and possibly a permanent night side" and so could be subject to "global glaciation and / or atmospheric collapse." If its atmosphere has more than 10 bars (10 times Earth's atmospheric pressure at sea level) of CO₂ and and other gases (such as molecular nitrogen, or N₂), however, three-dimensional climate simulations suggest that the planet's global mean temperatures could rise above freezing for land as well as ocean-covered surfaces (CNRS press release; Wordsworth et al, 2011; and Jon Voisey, Universe Today, May 6, 2011).

© LMD, CNRS

Larger and jumbo illustrations (source).

Model simulations suggest that
Gliese 581 d can be warm enough
to have liquid water on its surface
if its atmosphere is sufficiently
thick and rich in CO₂ (more).

© ESO, unknown artist



Larger illustration.




Gliese 581 now appears
to host at least six
planets between Earth
and Neptune in mass
(NASA 2010 news
and ESO 2007 news).

As of September 29, 2010, Gliese 581 is believed to host at least six planets between the size of Earth and Neptune, with one orbiting the star wholly 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 -- 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, 2008; 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, but revised orbital analysis and atmospheric modelling to indicate that planetary candidate "d" could have a deep global ocean and may not be as hospitable to land-based, Earth-type plant and animal life despite an orbit within the outer boundary of the habitable zone of Gliese 581 (ESO press release; and Mayor et al, 2009 -- more below). On September 29, 2010, a team of astronomers announced the discovery of a potentially habitable, rocky super-Earth candidate "g" orbiting wholly within the habitable zone of Gliese 581 as well as another larger but frigid super-Earth "f" in an outermost orbit (NASA news release; Carnegie news release; U.C. at Santa Cruz press release; NSF video announcement; Amir Alexander, Planetary News, October 6, 2010; David Shiga, New Scientist, September 29, 2010; Dennis Overbye, New York Times, September 29, 2010; Astronomy Picture of the Day; and Vogt et al, 2010 -- more below). On October 11, 2010, an astronomer from the Geneva Observatory's HARPS project announced at the IAU Symposium 276 that their team had not been able to confirm the existence of either planetary candidates Gliese 581 "g" or "f" based on only their own, expanded but smaller dataset of 180 observations over 6.5 years (Leslie Mullen, Astrobiology, October 12, 2010; and Rachel Courtland, New Scientist, October 13, 2010 -- more below).

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).

---------------------------------------------- [Guide] -- [Larger] ----------------------------------------------

	Orbital Distance (a=AUs)	Orbital Period (P=days)	Orbital Eccen. (e)	Orbital Inclin. (i=°)	Mass (Earths)	Diameter (Earths)	Surface Gravity (g)	Low-Hi. Temp. (°F)	Low-Hi. Temp. (°C)	Avg.Sur. Temp. (°K)	Avg.Equi. Temp. (°K)
Gliese 581	0.0	...	...	...	102,000	32	...	...	...	...	...
Planet "e"	0.0285	3.15	~0	40-85	1.86+/-0.41	>1	...	hot	hot	hot	420
Planet "b"	0.0406	5.37	~0	40-85	16.00+/-1.17	>4?	...	hot	hot	=>647	>320
Planet "c"	0.073	12.9	~0	40-85	5.30+/-0.09	=>1.5	1.3-2	=>	=>	=>647	~320
Inner H.Z. Edge?	0.11?	25.2	0.0	40-85	...	...	...	...	...	288	<270
Planet "g?"	0.134	36.1	~0	40-85	2.24+/-0.64	1.3-1.5	1.1-1.7	(29)-160	(34)-71	>242-261+	209-228
Planet "d?"	0.218	66.7	~0	40-85	5.94+/-1.05	=>1.5	=>1.8	cold?	cold?	>273	203-233
Outer H.Z. Edge?	0.28?	87.8	0.0	40-85	...	...	...	...	...	<273	<203
Planet "f?"	0.758	433	~0	40-85	=>7.0	>1.5	...	frigid	frigid	<203	<203

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, as small as 29 percent of its diameter (Chabrier and Baraffe, 2000; Pasinetti-Fracassini et al, 2001; and Johnson and Wright, 1983), and a bit more than one percent (around 0.0135) of its bolometric luminosity (Vogt et al, 2010; Bonfils et al, 2005, in pdf). The star appears to be around 32 to 96 percent as enriched as Sol in elements heavier than hydrogen (Rojas-Ayala et al, 2010, Table 1, page 16; Johnson and Apps, 2009; Bean et al, 2006; Ken Croswell, 2006; and Bonfils, et al, 2005).

Zina Deretsky, NSF



Larger and jumbo illustrations.



As of September 29, 2010, Gliese 581
appears to host at least six planets,
including one (planetary candidate g)
that orbits wholly within its relatively
narrow, red-dwarf habitable zone (more).

A relatively inactive star with a rotational period of about 94.2 +/- 1.0 days (Vogt et al, 2010, page 11), Gliese 581's 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 (Vogt et al, 2010; and Udry et al, 2007). Moreover, the Canadian Space Agency's MOST satellite monitored Gliese 581 for a month and a half and announced in 2007 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 581 has been previously classified as a variable star with the designation "HO Librae" and NSV 7023, but "its short-term variability to be at most 0.006 magnitudes" (Vogt et al, 2010; and Edward W. Weis, 1994). 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.

Planetary System

As of September 29, 2010, Gliese 581 was believed by some astronomers to host at least six planets between the size of Earth and Neptune, with one planetary candidate ("g") orbiting the star wholely within its habitable zone whose existence has not been confirmed.

© ESO, unknown artist






Larger illustration.






Fourth to be discovered, innermost
planetary candidate "e" with two
to three times Earth's mass was
first detected in 2009 (more).

Planet "e" - On April 21, 2009, a team of astronomers at the 2009 European Week of Astronomy and Space Science conference announced the discovery of planetary candidate "e" with a minimum mass initially estimated to be around 1.9 (and a maximum of 3.1) Earth-masses in an innermost orbit, along with new orbital analysis suggesting that planetary candidate "d" may orbit Gliese 581 in its so-called habitable zone (ESO press release; Mayor et al, 2009; Selsis et al, 2008); and von Bloh et al, 2008. The fourth object to be discovered around Gliese 581, planet e is now thought to have at least 1.7 Earth-masses (or 0.0053 Jupiter-masses). It revolves around Gliese 581 -- inside of planet b's orbit -- at an average distance of 0.0285 AU, in a roughly circular orbit (e~0) which it completes in 3.1494 +/- 0.0305 days (Vogt et al, 2012; Vogt et al, 2010; and Mayor et al, 2009).

JPL, NASA



Larger image.



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

Planet "b" - 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 14.8 times (maximum of 30.4) that of the Earth. With a semi-major axis of 0.041 AUs (around 6 million kilometers), it orbits so close to its host star that its orbital period lasts only 5.37 days, in a highly circular orbit (e~0) (Vogt et al, 2012; Vogt et al, 2010; Mayor et al, 2009; and Udry et al, 2007). As a result, the planet must be very hot, with an equilibrium temperature at around 297° Fahrenheit (147° Celsius or 420 kelvins).

© ESO, unknown artist





Larger illustration.





On Planet "c," water is unlikely to exist on its
day-side surface (except as lakes or seas on
its colder dark side opposite the star) under
a likely dense atmosphere (more).

Planet "c" - 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 5.3 (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 highly circular orbit (possibly not as eccentric as e= 0.17 ± 0.07) which it completes in 12.9 days (Vogt et al, 2012; Vogt et al, 2010; and Mayor et al, 2009). Based on its minimum mass and a range of predicted radii, the surface gravity of planet c may lie between 1.3 and 2 g (Selsis et al, 2008; Sotin et al, 2007; and Valencia et al, 2007).

Planet c was initially calculated to orbit Gliese 581 near the hot, inner edge of the red dwarf's relative narrow and very close habitable zone. It was thought to have a equilibrium temperature between -3° and more than 40° Celsius (27° and more than 104° Fahrenheit) with water on its surface, depending on whether the planet has a Venus- or an Earth-like albedo (Udry et al, 2007). Subsequent models indicated, however, that such a massive planet is likely have a dense atmosphere with a gas composition that should undergo a Venus-like runaway greenhouse effect, which should boil off all surface water (Vogt et al, 2010; Selsis et al, 2008; and von Bloh et al, 2008), although others have suggested that surface lakes or seas could be possible on the colder, perpetually dark side of planet c (see an illustration of such water features on planet c by Lynette Cook). (Illustrations of another artist's vision of day- and night-side surfaces of planet c due to tidally locked, synchronous rotation around host star Gliese 581 are available from Karen Wehrstein and Astronomy Picture of the Day.)

© ESO, unknown artist



Animation still.



Planetary candidate "d" orbits just within
the outer edge of Gliese 581's habitable
zone and so may have liquid surface
water in a deep global ocean (more).

Planet "d?" - On April 21, 2009, a team of astronomers at the 2009 European Week of Astronomy and Space Science conference announced revised orbital analysis suggesting that planetary candidate "d" may orbit Gliese 581 in its so-called habitable zone (ESO press release; Mayor et al, 2009; von Bloh et al, 2008; and Selssis et al, 2007). Planet "d" is now estimated to have between 5.9 and 8.4 Earth-masses. It revolves around Gliese 581 -- outside of planet b's orbit -- at an average distance of 0.218 AU, probably in a circular orbit (rather than a previously calculated eccentric orbit with e= 0.38 +/- 0.09) which it completes in 66.7 days, not quite a fifth of an Earth year. Although it was once thought possible that this detection was actually caused by a large star spot, its discoverers now believe that the host star is too inactive for this possibility to be likely (Vogt et al, 2012; Vogt et al, 2010; Mayor et al, 2009; and Udry et al, 2007). Subsequent calculations suggest that a greenhouse effect under the right atmospheric conditions may make it possible for liquid water to persist on its surface despite its tidally locked, synchronous rotation around Gliese 581 (von Bloh et al, 2008).

Orbiting near the outer edge of Gliese 581's habitable zone at an orbital distance of just under 0.22 AU, planet d receives "35% less stellar energy than Mars and is probably locked in tidal resonance, with extremely low insolation at the poles and possibly a permanent night side" and so could be subject to "global glaciation and / or atmospheric collapse" (Wordsworth et al, 2011). The planet would have a maximum equilibrium temperature of only around -94° Fahrenheit (-70° Celsius and 203 kelvins). if it had a Bond albedo (the fraction of power at all wavelengths re-radiated back into space) of 0, or -40° F (-40° C or 233 K) under more optimistic assumptions (Olaf Stampf, Der Spiegel, October 8, 2010; Kaltenegger et al, 2010; Vogt et al, 2010), which would be too cold to maintain liquid water on its surface. As planet d is calculated to have between 5.6 to 8.4 Earth-mass, however, it should be tectonically active with massive volcanic eruptions that spew lots of carbon dioxide (CO₂) and other greenhouse gases to create a dense envelope of air under high atmospheric pressure.

© LMD, CNRS

Larger and jumbo illustrations (source).

Model simulations suggest that
Gliese 581 d can be warm enough
to have liquid water on its surface
if its atmosphere is sufficiently
thick and rich in CO₂ (more).

Models show that when atmospheric pressure rises to at least five bar (5 times Earth's atmospheric pressure at sea level), the resulting greenhouse effect in a high-CO₂ atmosphere can cause temperatures to rise significantly above freezing and thaw surface ice (Olaf Stampf, Der Spiegel, October 8, 2010; Kaltenegger et al, 2010; von Paris et al, 2010; Wordsworth et al, 2010; Vogt et al, 2010; Selsis et al, 2008; and von Bloh et al, 2008). Model calculations suggest that atmospheric pressure on the planet may reach up to seven or eight bars, a level found on Earth at the bottom of lakes. If the atmosphere has more than 10 bars of CO₂ and and other gases (such as molecular nitrogen, or N₂), however, three-dimensional climate simulations suggest that the planet's global mean temperatures could rise above freezing for land as well as ocean-covered surfaces (Wordsworth et al, 2011; and Jon Voisey, Universe Today, May 6, 2011). Some astronomers also believe that planet d is too massive to be made only of rock; more likely, the planet is ice-rich (like Neptune or Uranus which agglomerated large ice layers of water, ammonia, and methane) before migratng closer to Gliese 581, and so its abundant ices could have been warmed enough to melt into a deep global ocean (ESO press release). If planet d does have rocky surfaces, its surface gravity in those areas would be around 1.8 g (2009 presentation slides from Kaltenegger et al, 2010?).