Gliese 581: Difference between revisions
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{{main|Gliese 581 c}} |
{{main|Gliese 581 c}} |
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Gliese 581 c is believed to be a rocky planet, of 50% greater radius than Earth and about five times Earth's mass, orbiting a bit outside of the [[habitable zone]] of its parent star.The mean blackbody surface temperature has been estimated to lie between -3 °C (for a Venus-like [[albedo]]) and 40 °C (for an Earth-like albedo),<ref name="udry"/> , however, the temperatures are probably much higher (about 500 degress Celsius) because of likely runaway greenhouse effect. Given the closer-orbiting Neptune-mass planet, the system has undergone [[planetary migration]] and Gliese 581 c may have formed beyond the [[Frost line (astrophysics)|frost line]], with a composition similar to icy bodies like Ganymede.{{Fact|date=April 2007}} Gliese 581 c completes a full orbit in just under 13 days. Gliese 581 c orbits one of the hundred closest stars to Earth. |
Gliese 581 c is believed to be a rocky planet, of 50% greater radius than Earth and about five times Earth's mass, orbiting a bit outside of the [[habitable zone]] of its parent star.The mean blackbody surface temperature has been estimated to lie between -3 °C (for a Venus-like [[albedo]]) and 40 °C (for an Earth-like albedo),<ref name="udry"/> , however, the temperatures are probably much higher (about 500 degress Celsius) because of likely runaway greenhouse effect. Given the closer-orbiting Neptune-mass planet, the system has undergone [[planetary migration]] and Gliese 581 c may have formed beyond the [[Frost line (astrophysics)|frost line]], with a composition similar to icy bodies like Ganymede.{{Fact|date=April 2007}} Gliese 581 c completes a full orbit in just under 13 days. Gliese 581 c orbits one of the hundred closest stars to Earth. |
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However, as with much exoplanet news released by the astronomical community, the vast majority of the above is merely speculation upon speculation. |
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The mass is a minimum possible mass. What is rarely stated with respect to exoplanets is that the derived mass is dependent on the inclination of the system, a matter always clearly expressed in the study of [[Spectroscopic Binaries]], from where the principles used in most exoplanet searches come, but given little if any mention in exoplanet reports and tables of data. The minimum mass is the one the orbiting object would have if the system's orbital plane was exactly in our line of sight, when would be at an inclination of 90 degrees. A similar argument also applies to the derived distance of exoplanets from the parent star, the Semi Major Axis, which itself is dependent on inclination. The upper limit is harder to define, although will likely not be too different given the inclination is large enough for the exoplanet to have been found via radial velocity variations. |
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This in itself is somewhat irrelevant, for even if the minimum value applied, five Earth masses is roughly one third to one quarter of a Neptune or Uranus mass. Such that even at the minimum mass the object qualifies more as a "mini-Neptune" rather than a "super-Earth". |
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Spectroscopic radial velocity techniques also derive the mass of any companion for "single line objects" as a ratio respective to the more massive object. As there is no way to directly measure the mass of Gliese 581 itself (as it is not a conventional eclipsing, astrometric or visual binary of relatively short orbital period, for example, nor a double lined spectroscopic binary), but only to estimate it from principles often founded more in assumption than fact, especially for the red dwarfs which are objects less well known than most other main sequence stars, the mass of Gliese 581 is itself therefore uncertain. |
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These many rarely reported assumptions that go towards the determination of exoplanet particulars mean that many hot Jupiters may well be cool brown dwarfs, many Neptunes super Jupiters, and most super Earths baby Neptunes. The term itself, "exoplanet", as opposed to the more accurate "substellar object/companion", is already emotive and biased. One quandary exercising the minds of theorists at present is the large number of eccentric orbit objects lying very close to their parent star, with some of these parent stars being billions of years old. Tidal forces should have circularized these orbits long ago, just as one side of the Moon predominantly faces the Earth, and many clever and interesting theories are being made up to explain this problem away. Little if any work appears to be being done to check if there are other interpretations of the data, not just that of a strangely large, incredibly hot, very massive object orbiting not only peculiarly close to its parent star, but doing so in an eccentric orbit, and apparently having done so for a very long time indeed. |
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Only the few transiting objects, where extra independent information and data can be gleaned, are well constrained with respect to their nature and details, although still liable to minimum mass and minimum separation caveats, as there too inclination affects the results. However, fortunately in those cases inclination has to be near the 90 degree, line of sight, level for the transits to be occuring in the first place. |
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Finally, the Goldilocks Scenario is calibrated against only one known system in the entirety of the Universe, specifically the Solar System, which is the only one adequately measured to exist as such. And within that stellar system's "habitable zone" lies Earth, an overbaked high pressure oven called Venus, and a rarefied fridge called Mars. |
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Another fairy tale is possibly more appropriate to current exoplanet studies, termed the "Emperor's New Clothes Effect". |
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A more accurate and interesting analysis of Gliese 581 c and its proximity to its parent star might be to ask if the atmospheric pressure and temperature is enough for the methane to fall as rain, sublimate out as a kind of snow, probably all altitude dependent, or simply always remain gaseous. |
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=== Gliese 581 d === |
=== Gliese 581 d === |
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Revision as of 10:49, 11 June 2007
 | This article documents a current event. Information may change rapidly as the event progresses, and initial news reports may be unreliable. The latest updates to this article may not reflect the most current information. (April 2007) |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Libra |
| Right ascension | 15h 19m 26.8250s |
| Declination | −07° 43′ 20.209″ |
| Apparent magnitude (V) | 10.56 |
| Characteristics | |
| Spectral type | M3V |
| B−V color index | 1.60 |
| Variable type | BY Draconis variable |
| Astrometry | |
| Radial velocity (Rv) | -9.5 km/s |
| Proper motion ( | RA: -1224.55 mas/yr Dec.: -99.51 mas/yr |
| Parallax ( | 159.52 ± 2.27 mas |
| Distance | 20.4 ± 0.3 ly (6.27 ± 0.09 pc) |
| Absolute magnitude (MV) | 11.56 |
| Details | |
| Mass | 0.31[1] M☉ |
| Radius | 0.38[1] R☉ |
| Luminosity | 0.013[2] L☉ |
| Temperature | 3,480[3] K |
| Metallicity | [M/H] = -0.33[3] |
| Age | 4.3 × 109[1] years |
| Other designations | |
| Database references | |
| SIMBAD | data |
Gliese 581 (IPA: [ˈgliːzə]) is an M2.5V red dwarf star located 20.40 light years away from Earth. It is about two degrees north of Beta Librae, the brightest star in the constellation Libra. Its mass is estimated to be approximately a third that of the Sun, and with respect to Earth, is the 87th closest star system.[4] The star system recently gained attention after Gliese 581 c, the first low mass extrasolar planet found to be in its star's habitable zone, was discovered in April 2007[5].
It has since been shown that Gliese 581 c will have a runaway greenhouse effect, and would not be habitable.[6] However, Gliese 581 d is near the outer edge of the Habitable zone. [7] Observations suggest that the star has at least three planets.
Star
The name Gliese 581 refers to the Star catalogue of nearby stars by Wilhelm Gliese. Other names of this star include BD-07° 4003 (BD catalogue, first known publication) and HO Librae (variable star designation). It does not have an individual name such as Sirius or Procyon.
Gl 581 is wrongly classified as a BY Draconis variable in the General Catalogue of Variable Stars, where the reference to the study of the star that led to its inclusion in the General Catalogue clearly shows no evidence of variability, least of all data indicative of BY Draconis variability. The data is well within the expected noise level of data discovered in this way[8]. Both the ASAS-3 data and Hipparcos Epoch Photometry show the star to be stable at at least 0.1 magnitudes both in amplitude and in mean magnitude over time, and no real evidence exists for classical variability having ever been seen in this star, least of all formal evidence of BY Draconis variability (such as a periodic sinusoidal light curve representing the motion of any starspot as it moves due to the stellar rotation).
An M-class dwarf star such as Gliese 581 has a much lower mass than the Sun, causing the core region of the star to burn hydrogen at a significantly lower rate. As a result, this star has only 1.3% of the Sun's total luminosity, and a planet would need to be situated much closer to this star in order to receive a comparable amount of energy as the Earth. The region of space around a star where a planet would receive roughly the same energy as the Earth is sometimes termed the "Goldilocks Zone", or, more accurately, the habitable zone.
Planetary system
At least three planets are believed to be orbiting this star. One, about Neptune-sized, was discovered in 2005.[2] Another, having an estimated radius 1.5 times that of Earth, was discovered in 2007.[9] The latter is notable as it is the smallest planet yet discovered in the habitable zone of another star, making it the most "earthlike" exoplanet found to date. Observations of the star also revealed the possibility of a third planet with a mass of roughly 8 Earths and an orbit of 84 Earth days.
Gliese 581 b
Gliese 581 b was the fifth planet to be discovered around a red dwarf star. This inner planet is 16 times as massive as Earth (similar to Neptune's mass) and completes a full orbit of Gliese 581 in only 5.4 days.
Gliese 581 c
Gliese 581 c is believed to be a rocky planet, of 50% greater radius than Earth and about five times Earth's mass, orbiting a bit outside of the habitable zone of its parent star.The mean blackbody surface temperature has been estimated to lie between -3 °C (for a Venus-like albedo) and 40 °C (for an Earth-like albedo),[9] , however, the temperatures are probably much higher (about 500 degress Celsius) because of likely runaway greenhouse effect. Given the closer-orbiting Neptune-mass planet, the system has undergone planetary migration and Gliese 581 c may have formed beyond the frost line, with a composition similar to icy bodies like Ganymede.[citation needed] Gliese 581 c completes a full orbit in just under 13 days. Gliese 581 c orbits one of the hundred closest stars to Earth.
However, as with much exoplanet news released by the astronomical community, the vast majority of the above is merely speculation upon speculation.
The mass is a minimum possible mass. What is rarely stated with respect to exoplanets is that the derived mass is dependent on the inclination of the system, a matter always clearly expressed in the study of Spectroscopic Binaries, from where the principles used in most exoplanet searches come, but given little if any mention in exoplanet reports and tables of data. The minimum mass is the one the orbiting object would have if the system's orbital plane was exactly in our line of sight, when would be at an inclination of 90 degrees. A similar argument also applies to the derived distance of exoplanets from the parent star, the Semi Major Axis, which itself is dependent on inclination. The upper limit is harder to define, although will likely not be too different given the inclination is large enough for the exoplanet to have been found via radial velocity variations.
This in itself is somewhat irrelevant, for even if the minimum value applied, five Earth masses is roughly one third to one quarter of a Neptune or Uranus mass. Such that even at the minimum mass the object qualifies more as a "mini-Neptune" rather than a "super-Earth".
Spectroscopic radial velocity techniques also derive the mass of any companion for "single line objects" as a ratio respective to the more massive object. As there is no way to directly measure the mass of Gliese 581 itself (as it is not a conventional eclipsing, astrometric or visual binary of relatively short orbital period, for example, nor a double lined spectroscopic binary), but only to estimate it from principles often founded more in assumption than fact, especially for the red dwarfs which are objects less well known than most other main sequence stars, the mass of Gliese 581 is itself therefore uncertain.
These many rarely reported assumptions that go towards the determination of exoplanet particulars mean that many hot Jupiters may well be cool brown dwarfs, many Neptunes super Jupiters, and most super Earths baby Neptunes. The term itself, "exoplanet", as opposed to the more accurate "substellar object/companion", is already emotive and biased. One quandary exercising the minds of theorists at present is the large number of eccentric orbit objects lying very close to their parent star, with some of these parent stars being billions of years old. Tidal forces should have circularized these orbits long ago, just as one side of the Moon predominantly faces the Earth, and many clever and interesting theories are being made up to explain this problem away. Little if any work appears to be being done to check if there are other interpretations of the data, not just that of a strangely large, incredibly hot, very massive object orbiting not only peculiarly close to its parent star, but doing so in an eccentric orbit, and apparently having done so for a very long time indeed.
Only the few transiting objects, where extra independent information and data can be gleaned, are well constrained with respect to their nature and details, although still liable to minimum mass and minimum separation caveats, as there too inclination affects the results. However, fortunately in those cases inclination has to be near the 90 degree, line of sight, level for the transits to be occuring in the first place.
Finally, the Goldilocks Scenario is calibrated against only one known system in the entirety of the Universe, specifically the Solar System, which is the only one adequately measured to exist as such. And within that stellar system's "habitable zone" lies Earth, an overbaked high pressure oven called Venus, and a rarefied fridge called Mars.
Another fairy tale is possibly more appropriate to current exoplanet studies, termed the "Emperor's New Clothes Effect".
A more accurate and interesting analysis of Gliese 581 c and its proximity to its parent star might be to ask if the atmospheric pressure and temperature is enough for the methane to fall as rain, sublimate out as a kind of snow, probably all altitude dependent, or simply always remain gaseous.
Gliese 581 d
Gliese 581 d is about eight times as massive as the Earth and orbits its sun in 84 days.It orbits on the outer edge of the habitable zone of it's star, and it is a lot more likely candidate for habitability for at least a primitive life forms than Gliese 581 c[10] .
See also
References
- ^ a b c "Star: Gl 581". Extrasolar Planets Encyclopaedia. Retrieved 2007-04-26.
- ^ a b Bonfils; et al. (2005). "The HARPS search for southern extra-solar planets, VI. A Neptune-mass planet around the nearby M dwarf Gl 581". Astronomy and Astrophysics. 443: L15–L18.
{{cite journal}}: Explicit use of et al. in:|author=(help) - ^ a b Bean, J. L.; Benedict, G. F.; Endl, M. (2006). "Metallicities of M Dwarf Planet Hosts from Spectral Synthesis". The Astrophysical Journal. 653 (1): L65–L68. Retrieved 2007-02-04.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ "The One Hundred Nearest Star Systems". RECONS. Retrieved 2007-05-29.
- ^ Than, Ker (2007). "New Planet Could Harbor Water and Life". SPACE.com. Retrieved 2007-05-29.
- ^ "The Habitability of Super-Earths in Gliese 581" (PDF). Retrieved 2007-05-29.
- ^ "The Habitability of Super-Earths in Gliese 581" (PDF). Retrieved 2007-05-29.
- ^ "BY Draconis variables". Sternberg Astronomical Institute, Moscow. Retrieved 2007-06-11.
- ^ a b Udry; et al. (2007). "The HARPS search for southern extra-solar planets, XI. An habitable super-Earth (5 M⊕) in a 3-planet system" (PDF). Astronomy and Astrophysics. preprint: preprint.
{{cite journal}}: Explicit use of et al. in:|author=(help) - ^ "The Habitability of Super-Earths in Gliese 581" (PDF). Retrieved 2007-05-29.
- "Major Discovery: New Planet Could Harbor Water and Life"
- Sol Station: Gliese 581 / HO Librae
- BBC News report
- CTV News Report
External links
- Extrasolar Planets Encyclopaedia: Gl 581
- SIMBAD: V* HO Lib -- Variable Star
- All Wet? Astronomers Claim Discovery of Earth-like Planet, Scientific American
- Position of Gliese 581 marked on local space. (Top right corner)
- Photo of Constellation Libra with Gliese 581
- Artist conceptions of planets of Gliese 581
- GLIESE 581 c (french)
- Speculation about geology/geochemistry of Gliese 581c The Geochemical Society
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