Gliese 581 System

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Exoplanetary Scratchpad

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Small nearby Red Dwarf with six planets in tight circular orbits and a distant Kuiper belt where many comets orbit. Gliese 581 e was, at the time of its discovery, the smallest known dopplar-detected exoplanet and a super-Mercury, b is a hot-Neptunian, c is a super-Venus and the first detected in the habitable zone (initially heralded as habitable, but later thought too hot due to the greenhouse effect), g is a super-Earth and the first detected in the middle of the HZ (and is highly controversial, having many doubters and defenders), d is a super-Earth on the outer edge of the HZ which could support liquid water (due to its presumably large atmospheric pressure and carbon dioxide), and f (its existence is also highly controversial) is a cold super-Earth. Much further out, from 25 ± 12 AU to more than 60 AU, there is a cold debris disk reminiscent of the Kuiper belt but with 10 times more comets than the one in our solar system. The star is not very active.

Gliese 581 System Web Pages

Gliese 581 System In the News

SETI's Observations (1995, 1997)

Neptunian Planet b Discovered (Nov 2005)

The discovery of a Neptunian planet around this nearby Red Dwarf suggests such planets are common and only now detectable.

Super-Earth Discovered in Habitable Zone (Apr 2007)

The first terrestrial planet discovered that is potentially capable of hosting liquid water and possibly life. Also the smallest planet discovered to date around a normal star. Another neptunian also discovered further out.

Planet c More Habitable than d (May/Jun 2007)

No Transits Detected (Jun 2007)

Planet c Probably Never Inhabitable (Sep 2007)

Smallest Exoplanet e Discovered (Apr 2009)

Bebo Transmissions to the Planet (Aug 2009)

First Habitable Planet g Found (Sep 2010)

Doubts Over Planet g (Oct 2010)

Doubts Over Planet g (Nov 2010)

Planet d May be in Habitable Zone (Dec 2010)

How to Study Atmospheres in Planets Like Gliese 581 g (Dec 2010)

Doubts Over Planet g (Jan 2011)

Another Team Doubts Planet g (Feb 2011)

Vogt Defends Planet g (Feb 2011)

Liquid Water Would Be Stable on Planet D (Jun 2011)

Also, transits not found for planet e and star found to be stable.

Planet D Prime Candidate in the Habitable Exoplanet Catalog (Dec 2011)

Appearance Rendered (Dec 2011)

Debris Disk "Exo-Kuiper Belt" (Nov 2012)


The fifth planetary system on the tour is one of the most exciting, which is Gliese 581. It is a red dwarf star that contains six known super-earth and Neptunian exoplanets, including the first potentially habitable exoplanet known. Its planets range from epistellar distances to Venus-like distances and from at least 2 Earth Masses to a Neptune-like mass. The system can be compared to the Gliese 876 system, a similarly small BY Draconis variable Red Dwarf with multiple planets. The size of its planets are smaller than those of that system though.

The star itself is 20.3 light years away in the constellation Libra, the scales. This is the fifth nearest planetary system known, the fourth nearest Red Dwarf system, and the 117th nearest star. One of the nearest star systems to it is Gliese 570, a triple star system 5 light years away, also in Libra, that contains one orange dwarf, two red dwarves, and one of the coolest known brown dwarves. On October 3 2008, shortly after discovery of Planet c, radio transmissions were sent to the star. The transmission originated from an observatory in the Ukraine and contains 501 messages selected by a contest through the social network web site Bebo. This transmission will take 20 years to arrive and will do so in early 2029. Any responses would return another twenty years from then in 2049. Seti has monitored the stars for signals and has not detected any.

Main Star a

The star is about a third of the mass of the Sun. It is a smaller star than any of the other stars closer than it that harbor known planets, though not by much. The star is about 7 to 11 Billion years old, which is roughly twice as old as the Sun. It is about one percent as luminous as the Sun is. Like the nearer Gliese 876, it is a BY Draconis variable star and also known as HO Librae. These stars vary in brightness as they rotate, with star spots and other activities coming into view. It could emit occasional x-ray flares that would affect any planets around it. Like the other nearby Red Dwarves, this was cataloged by Wilhelm Gliese in 1957.

Planet e

The first planet in the tour is the nearest one to its star, which is planet e, the fourth discovered planet. This is the smallest known planet in the system at about twice Earth's mass. It is also currently smallest mass known exoplanet around a normal star. This beat the record previously held by MOA-2007-BLG-192L b, a frozen microlensing-detected planet around a particularly small star, which is three times as massive as the Earth.

Planet e was discovered in 2009, in the third batch of planetary discoveries. Its discovery coincided with an orbital adjustment of its outermost known planet d was closer in than previously thought and more likely to have liquid water. It orbits about 3% Earth's distance, somewhat closer than 51 Pegasus b to its star. It is not as close as more nearby Gliese 876 d is from its star, however, which is 2% Earth's distance from the Sun and was also once known as the smallest known exoplanet and the first super-Earth discovered.

Neither of the two transiting super-Earths found so far, CoRoT-7b or GJ 1214b, make good analogs for this planet. Both are significantly more massive (about 8 Earth masses). CoRoT-7b is a much hotter place, with temperatures hot enough keep the surface molten from solar radiation, and where silicate sand rains down on the surface. None of these processes likely occur on Gliese 581b. GJ 1214b is at a temperature more similar to Gliese 581, but has a large diameter for its mass, which could indicate that it is a mini-Neptune, a terrestrial with a large atmosphere due to outgassing, or a hot ocean world with water "compressed" by a thick gaseous envelope. Gliese 581 may not be massive enough to maintain an atmosphere capable of inflating the planet in this manner. The exoplanet can likely be considered a super-Mercury. Like CoRoT-7b and Mercury, it likely has little atmosphere, with much of it having escaped the planet. Like CoRoT-7b and GJ 1214b, it is tidally locked to its star and likely has undergone great tidal stress, which causes Io-like volcanism.

Standing on its surface, its star would appear ten times as large as the Sun does from Earth. It would remain in the same spot in the sky all the time, and the constellations of this star's zodiac could be seen passing behind the star on this airless world in regular 3 day intervals. The Neptune-sized planet b would loom as large as a moon would, which can get within five times the distance the Earth is from the Moon. On the eternal night side of the planet, planet b may be the best source of light, which would appear regularly every few days. The planets, which all have perfectly circular orbits and likely perfect inclinations to each other, would perform regular transits with each other and pass behind the star. They would follow the same path through the sky repeatedly as they more along in their orbits. One could also look up at our Sun and see it as a yellow star without the aid of binoculars. Neptune could be detected from there using current dust analysis methods.

Planet b

Planet b is the next planet out. Though it is the largest in the system, it was one of the first planets as small as Neptune that was discovered. Its minimum mass is 16 that of Earths, which is in between Uranus and Neptune. It does not dominate the system as Jupiter does the solar system. It is about twice as massive as the second most massive planet, the outermost planet f and composes about 40% of the mass of the known planets around the system. It is the smaller than any of the other "largest" planets that are closer to the sun than it is.

A transiting analog for this planet is Gliese 436 b, the smallest and nearest transiting Hot Neptune. The analog planet was found to have a higher temperature than expected and a larger radius. A layer of "hot ice" compressed by a thick atmosphere and an outer layer of Hydrogen and Helium may account for it. It also was found to have a mysterious methane deficiency and carbon monoxide abundance. That Neptunian is a little larger though, orbits a little further in, orbits a hotter red dwarf, and is thus hotter in general. The discovery of this planet around a star so nearby in 2005 showed that Neptune sized planets are likely common. It was discovered by the Geneva team using the HARPS instrument at Chile, which would also announce planets c, d, and e. At the time of its discovery, five dopplar-detected exoplanets had potentially lower masses. It was only the fifth planet found around a Red Dwarf, after the three known planets around Gliese 876 and the one around Gliese 436. Artist concepts of this planet is a blue hot Neptune-like planet.

Planet c

Planet c is the third planet from the star. This planet's discovery was heralded as the first rocky planet found in the habitable zone. Its discovery prompted radio signals to be sent towards the star. While it is in the habitable zone, it is on the inner edge of this zone. It was found that the planet is also susceptible to developing a runaway green-house gas like Venus, but conversely, the outer planet d was now considered at the outer edge of the habitability zone. Once thought to be the smaller of the two, the planet is actually a mass twin of its counterpart on the other side of the habitable zone, planet d.

A study was performed to determine if perhaps planet c once orbited further out and in the habitability zone, but then migrated inwards due to stellar tidal effects, which are capable of moving planets over the time span of a billion years. It was found that for in order for this to have happened, the planet would have crossed the 3:1 mean motion resonance point with the more inner Neptunian planet b, which would have resulted in a resonance capture between these planets. Since they are not in 3:1 resonance, planet c was probably never far away enough from its star to have been habitable in its past.

Planet b's closest transiting analog is GJ 1214b, which is somewhat closer to its star than the habitable zone and about one earth mass more massive. This planet shows that exoplanets about as massive as this can get to be quite large and possibly maintain a thick atmosphere. Planet c could be similar, with a large atmosphere. Even if greenhouse gasses caused the temperature to go above the boiling point, a thick atmosphere could force the water into a hot ocean state.

The planet was also the smallest known exoplanet around a sun-like star at the time of its discovery, just over 5 Earth masses, though it is now thought to be just under 6 Earths. This beat the two year old previous record held by the first icy super-Earth discovered, OGLE-2005-BLG-390L b, which was found with microlensing. It was thought to have lost its status the following year to Gliese 436 c, which is also nearby and found to be transiting, but this was later retracted. It did lose its status to MOA-2007-BLG-192L b, which would lose its status to another planet in this system, planet e.

Planet g

Planet g was announced as the first almost certainly habitable planet found around another star and the first planet found to be in the center of the habitability zone. It is 3 to 4 times as massive as the Earth and perhaps 30% larger to twice as large in diameter. Any larger than that and the system would lose stability. This is small enough to have a solid surface and also not too crushing of an atmosphere. Its surface gravity would also be large enough to support a decent sized Earth-like atmosphere. Like its neighbors, the planet is tidally locked, with one side under eternal sunlight, while the other in total darkness. In one report, models show that the hottest point is not hot enough for water to boil completely away, while the coldest point is not enough for it to freeze out. Winds from the light to dark side of the planet are relatively benign. In another report, the average temperature would be below freezing, but the heat distribution would allow some parts of it to be warm enough for liquid water. A temperate region in its twilight zone is the most likely part to have liquid water, and thus the best spot to have life. Any lifeforms would probably evolve tailored to flourish in a given longitude.

The planet was announced with the outermost planet f by the Lick-Carnegie team using the HIRES instrument in Hawaii. The planet could not be confirmed by the Swiss team which had discovered the earlier announced planet, however. Further observations are needed to fully confirm these planets' existence. Planet g was announced at a time when the Kepler space telescope had data on several similarly habitable planets and expected to make the first such announcement, but because they were orbiting hotter, more sun-like stars, their orbit periods were more earth-like, and they still had to wait a few years to be able to confirm them. Its discovery was all the more exciting compared to what Kepler may discover due to its close proximity to Earth. There are currently no transiting analogs for this planet, but there will probably be some after Kepler's results come in, though these may not be tidally locked in the same way. The closest well studied planet to this one is in fact the Earth.

After the announcement of planet g, Australian SETI scientist Ragbir Bhathal claimed to have detected to have detected a suspicious pulse of light nearly two years ago that came from the same area of the galaxy as the location of Gliese 581g. Francis Drake, of the famous Drake equation, found his claims suspicious, as he was not allowed to see the data and there has been no repeated detections of this phenomenon.

Planet d

Planet d is a large Super-Earth on the outermost parts of the habitable zone and the outermost of the known tidally locked planets. It orbits its star a little further than half of Mercury's distance from the sun, but its orbit period is a little longer than Mercury's. When planet c captured headlines as a potentially habitable planet, planet d was thought to be too distant from its star to support liquid water and maybe too big to be Earthlike. Further studies on both planets found that the greenhouse effect on planet d actually puts it in the outer part of the habitability zone, while the same effects make planet c too hot to support liquid water. After the innermost planet e was discovered, its orbit was refined and it was found to be a little closer. After Planet e's discovery, Stephane Udry said "It is probably too massive to be made only of rocky material, but we can speculate that it is an icy planet that has migrated closer to the star. 'd' could even be covered by a large and deep ocean — it is the first serious 'water world' candidate." After the discovery of planets g and f on either side of it, its estimated mass was reduced from seven Earth masses to under six Earth masses, giving it the same mass as planet c on the inner edge of the habitability zone.

Planet f

Planet f is the second largest planet, at about half the mass of Neptune and planet b or 7 times the mass of Earth. It orbits much further away than the other known planets in the system, and is the only one at a distance similar to one of the Solar System's planets. There is enough space between it and the next planet in, d, for perhaps two small undetected planets or an asteroid belt. Though it orbits at Venus-like distances, it is a frigid world where only solid ice could exist. Given its star's weak gravity, its orbit period is larger than the Earths, taking 433 days. It likely has a thick atmosphere and it is uncertain if it has a solid surface or not. It was announced in 2010 during the fourth batch of planets discovered around this star at the same time as the "Goldilocks" planet g was found by the Lick-Carnegie team using the HIRES instrument in Hawaii. The planet could not be confirmed by the Swiss team which had discovered the earlier announced planets, however.

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