COSMIC SECRETS
Extra Solar Planets
New Planets Discovered
Gliese 581c
 The Earth-like Planet Gliese 581c
..
Artist's impression of the five-Earth mass planet, Gliese 581c, found in the habitable zone around the red dwarf Gliese 581, with the instrument HARPS on the ESO 3.6-m telescope. Credit: ESO
Gliese 581 c  or Gl 581 c is a "Super-Earth", a large extrasolar terrestrial planet, orbiting the red dwarf star Gliese 581.[3]Assuming that the planet's mass is close to the lower limit determined by radial velocity measurements (the true mass is unknown), it was the smallest known extrasolar planet around a main sequence star, but on April 21, 2009, another planet orbiting Gliese 581, Gliese 581 e, was announced with an approximate mass of 1.9 earth masses, which is now the smallest known extrasolar planet around a main sequence star.[4] Gliese 581 c generated interest because it was initially reported to be the first potentially Earth-like planet in the habitable zone of its star, with a temperature right for liquid water on its surface, and by extension, potentially capable of supporting extremophile forms of Earth-like life.[3][5] However, further research on the potential effects of the planetary atmosphere casts doubt upon the habitability of Gliese 581 c and indicates that the fourth planet in the system, Gliese 581 d, is a better candidate for habitability.[1][6][7] In astronomical terms, the Gliese 581 system is relatively close to Earth, at 20.3 light years (192 trillion km or 119 trillion miles) in the direction of the constellation of Libra.[8][9] This distance, along with the declination and right ascension coordinates, give its exact location in our galaxy. It is identified as Gliese 581 by its number in the Gliese Catalogue of Nearby Stars; it is the 87th closest known star system to the Sun.[10]

Contents:

  1. Discovery
  2. Physical characteristics
    1. Mass
    2. Radius
    3. Orbit
    4. Tidal lock
  3. Habitability and Climate
    1. Effective Temperatures
    2. Liquid water
    3. Tidally-locked models
  4. Future observations
  5. See also
  6. References
  7. Further reading
    1. News media reports
    2. Non-news media
Discovery

The discovery of the planet by the team of Stéphane Udry University of Geneva's Observatory in Switzerland was announced on April 24, 2007. [3]The team used the HARPS instrument (an echelle spectrograph) on the European Southern Observatory 3.6 m Telescope in La Silla, Chile, and employed the radial velocity technique to identify the planet's influence on the star. The Canadian-built MOST space telescope was used to conduct a follow-up study over the next six weeks. No transit was detected over this time, so a direct measurement of the planet has not yet been possible; however, the star's apparent magnitude changed very little, indicating that it provides a stable source of light and heat to Gliese 581 c.[11]
The team released a paper of their findings dated April 27, 2007, published in the July, 2007 journal Astronomy and Astrophysics. [1]In the paper they also announced the discovery of another planet in the system, Gliese 581 d, with a minimum mass of 7.7 Earth masses and a semi-major axis of 0.25 astronomical units.

Physical characteristics

Mass

The existence of Gliese 581 c and its mass have been measured by the radial velocity method of detecting extrasolar planets. The mass of a planet is calculated by the small periodic movements around a common centre of mass between the host star Gliese 581 and its planets. Because the "wobbling" of Gliese 581 is a result of all planets in its system, the calculation of the mass of Gliese 581c depends on the presence of other planets in the Gliese 581 system and on the inclination of the orbital plane with respect to Earth. Using the known minimum mass of the previously detected Gliese 581 b, and assuming the existence of Gliese 581 d, Gliese 581 c has a mass at least 5.073 times that of Earth.[2] The mass of the planet cannot be very much larger than this or the system would be dynamically unstable.[1] Dynamical simulations of the Gliese 581 system which assume the orbits of the planets are coplanar indicate that for inclinations less than about 10° the system would be unstable.[2] For Gliese 581 c, this corresponds to an upper bound of about 29 Earth masses, or about 70% more massive than Neptune.[12]

Radius

Since Gliese 581 c has not been detected directly, there are no measurements of its radius. Furthermore, the radial velocity method used to detect it only puts a lower limit on the planet's mass, which means theoretical models of planetary radius and structure can only be of limited use. However, assuming a random orientation of the planet's orbit, the true mass is likely to be close to the measured minimum mass.

Assuming that the true mass is the minimum mass, the radius may be calculated using various models. For example, if Gliese 581 c is a rocky planet with a large iron core, it should have a radius approximately 50% larger than that of Earth, according to Udry's team.[1][13] Gravity on such a planet's surface would be approximately 2.24 times as strong as on Earth. However, if Gliese 581 c is an icy and/or watery planet, its radius would be less than 2 times that of Earth, even with a very large outer hydrosphere, according to density models compiled by Diana Valencia and her team for Gliese 876 d.[14] Gravity on the surface of such an icy and/or watery planet would be at least 1.25 times as strong as on Earth. They claim the real value of the radius may be anything between the two extremes calculated by density models outlined above.[15]

..
Scale comparison of the relative sizes of the Earth and Gliese 581c, assuming Gliese 581c is a rocky body with a mass close to the minimum mass determined by the radial velocity method. Credit: NASA
Other scientists' views differ. Sara Seager at MIT has speculated that Gliese 581 c and other five-Earth-mass planets could be:[16]
    "rock giants" mostly of silicate.
    "cannonball" planets of solid iron.
    "gas dwarfs" mostly of helium and hydrogen.
    carbon-rich "diamond worlds"
    purely hot ice-VII worlds.
    purely carbon-monoxide worlds.


If the planet transits the star as seen from our direction, the radius should be measurable, albeit with some uncertainty. Unfortunately, measurements made with the Canadian-built MOST space telescope indicate that transits do not occur.[11]

For comparison, the smallest confirmed diameter of a planet around a main-sequence star is that of COROT-7b, which is about 70% larger than Earth.

Orbit

..
SVG image of the Gliese 581 system orbits. The orbital parameters are taken from Udry et al. (2007), Astronomy & Astrophysics 469, L43–L47 "The HARPS search for southern extra-solar planets XI. Super-Earths (5 and 8 M_⊕) in a 3-planet system", specifically the free eccentricity solution in Table 1 (found on page L45).

Note: This figure omits the most recently discovered planet in the system, Gliese 581 e .

Assumptions:

  1. It is assumed that the system is coplanar. This cannot be constrained by the radial velocity observations used to detect the planets in this system, but our solar system's planets are nearly coplanar, and both the systems of PSR B1257+12 (planets B and C) and Epsilon Eridani (planet b and the circumstellar disk) are coplanar, see Konacki and Wolszczan (2003), The Astrophysical Journal 591, L147–L150 "Masses and Orbital Inclinations of Planets in the PSR B1257+12 System" and Beust et al. (2006), The Astrophysical Journal 132, 2206–2218 "The Extrasolar Planet ɛ Eridani b: Orbit and Mass respectively, so this assumption is at least reasonable.
  2. The direction of rotation around the star is assumed to be the same for each planet. Again, this cannot be constrained by the current radial velocity observations, but is a reasonable assumption since this is true for the major planets in our own solar system, and is predicted by current theories of planet formation. Under these first two assumptions, the orientation of the orbits relative to each other is correct as depicted in this diagram, however their actual orientation in 3D space with respect to external reference points (e.g. the Sun, the galactic centre) is unknown, hence such directions are not indicated on the diagram.
  3. A further assumption is that the true masses of the planets are small compared to that of the star. If the masses were comparable to that of the star, the actual semimajor axis corresponding to the measured orbital period would be greater. This assumption is supported both by probability (the chance that we are observing a system which has sufficiently low inclination for this to be relevant is very low) and by dynamical stability arguments which indicate that the planets cannot have true masses much more than 6 times their minimum masses: see Beust et al. (2008), Astronomy & Astrophysics 479, 277–282 "Dynamical evolution of the Gliese 581 planetary system". For the most massive planet (Gliese 581 b) this corresponds to 10-3 times the stellar mass, implying a semimajor axis increase of about 1 part in 10000 over the case for minimum mass planets. This effect can therefore safely be neglected.
Interpreting the diagram

The planetary orbits are drawn in orthographic projection, as viewed from directly above the plane of the system so that the orbital direction is anticlockwise. The dashed lines are drawn between the star and the periastron point of each planet, in order to depict the relative orientation of the orbits with respect to each other. The position of each planet in its orbit is calculated using the time of periastron in the table in the Udry paper, and are drawn at a time chosen to be close to the start of the Udry et al. radial velocity observations, which are available here. This time is chosen to minimise the effects of both uncertainties in the orbital parameters and potential orbital evolution of the system.
While the orbits are drawn to the correct scale, the star and the planets themselves are not shown to scale as they would be too small to see on the scale of this diagram. In addition, the true radii of the planets themselves are currently unknown as no direct observation of the planets has yet been made.

The orbits of the Gliese 581 planetary system, prior to the discovery of Gliese 581 e. In the picture, Gliese 581 c is the second planet from the star. Gliese 581 c has an orbital period ("year") of 13 Earth days[8] and its orbital radius is only about 7% that of the Earth, about 11 million km[17], while the Earth is 150 million kilometres from the Sun[18]. Since the host star is smaller and colder than the Sun—and thus less luminous—this distance places the planet on the "warm" edge of the habitable zone around the star according to Udry's team.[1][13] Note that in astrophysics, the "habitable zone" is defined as the range of distances from the star at which a planet could support liquid water on its surface: it should not be taken to mean that the planet's environment would be suitable for humans, a situation which requires a more restrictive range of parameters. A typical radius for an M0 star of Gliese 581's age and metallicity is 0.00128 AU[19], against the sun's 0.00465 AU. This proximity means that the primary star should appear 3.75 times wider and 14 times larger in area for an observer on the planet's surface looking at the sky than the Sun appears to be from Earth's surface.

Tidal lock

Because of its small separation from Gliese 581, the planet has been generally considered to always have one hemisphere facing the star (only day), and the other always facing away (only night), or in other words being tidally locked.[20][21] Even then, the planet would undergo violent tidal flexing, because the orbital eccentricity is between 0.10 and 0.22.[2] Because tidal forces are stronger when the planet is close to the star, eccentric planets are expected to have a rotation period which is shorter than its orbital period, also called pseudo-synchronization.[22] An example of this effect is seen in Mercury, which is tidally locked in a 3:2 resonance, completing three rotations every two orbits. In any case, even in case of 1:1 tidal lock, the planet would undergo libration and the terminator would be alternatively lit and darkened during libration.[23]

Models of the evolution of the planet's orbit over time suggest that heating resulting from this tidal locking may play a major role in the planet's geology. Models proposed by scientists predict that tidal heating could yield a surface heat flux about three times greater than the Jupiter's moon Io's, which could result in major geological activity such as volcanoes and plate tectonics.[24]

Habitability and Climate

The study of Gliese 581 c by the von Bloh et al. team has been quoted as concluding "The super-Earth Gl 581c is clearly outside the habitable zone, since it is too close to the star."[7] The study by Selsis et al. claims even "a planet in the habitable zone is not necessarily habitable" itself, and this planet "is outside what can be considered the conservative habitable zone" of the parent star, and further that if there was any water there and it was lost when the red dwarf was a strong X-ray and EUV emitter, it could have surface temperatures ranging from 700 K to 1000 K (427 to 727 °C).[6] Temperature speculations by other scientists are based on the temperature of (and heat from) the parent star Gliese 581 and have been calculated without factoring in the wide margin of error (96 °C/K) for the star's temperature of 3432 K to 3528 K.[25]

Effective Temperatures

Using the measured stellar luminosity of Gliese 581 of 0.013 times that of our Sun, it is possible to calculate Gliese 581 c's effective temperature a. k. a. black body temperature. (note: this probably differs from its surface temperature). According to Udry's team, the effective temperature for Gliese 581 c, assuming an albedo (reflectivity) such as Venus' (0.64), would be −3 °C (26.6 °F), and assuming an Earth-like albedo (0.296), then it would be 40 °C (104 °F),[1][8] a range of temperatures which overlaps with the range that water would be liquid at a pressure of 1 atmosphere. However, the effective temperature and actual surface temperature can be very different due to the greenhouse properties of the planetary atmosphere: for example, Venus has an effective temperature of 34.25 °C (93.65 °F), but a surface temperature of 463.85 °C (866.93 °F), a difference of about 430 °C (774 °F).[26] Studies of the habitability of Gliese 581's planets[7][27] conclude that Gliese 581 c is likely to suffer from a runaway greenhouse effect similar to that found on Venus, as such, is highly unlikely to be habitable. Nevertheless, this runaway greenhouse effect could be prevented by the presence of sufficient reflective cloud cover on the planet's day side.[28] Alternatively, if the surface were covered in ice, it would have a high albedo (reflectivity), and thus could reflect enough of the incident sunlight back into space to render the planet too cold for habitability, although this situation is expected to be unstable except for very high albedos greater than about 0.95: release of carbon dioxide by volcanic activity or of water vapor due to heating at the substellar point would trigger a runaway greenhouse effect.[29]

Liquid water

..
By refining the orbit of the planet Gliese 581 d, first discovered in 2007, a team of astronomers has shown that it lies well within the habitable zone, where liquid water oceans could exist. This diagram shows the distances of the planets in the Solar System (upper row) and in the Gliese 581 system (lower row), from their respective stars (left). The habitable zone is indicated as the blue area, showing that Gliese 581 d is located inside the habitable zone around its low-mass red star.
Based on a diagram by Franck Selsis, Univ. of Bordeaux. - Credit ESO

Planetary habitable zones of the Solar System and the Gliese 581 system compared.

Gliese 581 c is likely to lie outside the habitable zone.[7][30] No direct evidence has been found for water to be present, but it is probably not present in the liquid state. Techniques like the one used to measure the extrasolar planet HD 209458 b may in the future be used to determine the presence of water in the form of vapor in the planet's atmosphere, but only in the rare case of a planet with an orbit aligned so as to transit its star, which Gliese 581 c is not known to do.

Tidally-locked models

Theoretical models predict that volatile compounds such as water and carbon dioxide, if present, might evaporate in the scorching heat of the sunward side, migrate to the cooler night side, and condense to form ice caps. Over time, the entire atmosphere might freeze into ice caps on the night side of the planet. Alternatively, an atmosphere large enough to be stable would circulate the heat more evenly, allowing for a wider habitable area on the surface.[32] For example, although Venus has a small axial inclination, very little sunlight reaches the surface at the poles. A slow rotation rate approximately 117 times slower than Earth's produces prolonged days and nights. Despite the uneven distribution of sunlight cast on Venus at any given time, polar areas and the night side of Venus are kept almost as hot as day by globally circulating winds.[32] However, it remains unknown if water and/or carbon dioxide are even present on the surface of Gliese 581c.

Future observations

Gliese 581 c presents several challenges for study. It has not been directly observed, and the development of equipment sensitive enough to look for signs of (extremophile forms of) life will take years.[33] However, according to the research-team member Xavier Delfosse:

"Because of its temperature and relative proximity, this planet will most probably be a very important target of the future space missions dedicated to the search for extremophile forms of extraterrestrial life. On the treasure map of the universe, one would be tempted to mark this planet with an  X. "[13][33]
Astronomers Stéphane Udry, Dimitar Sasselov and Glenn White suggested that the earthlike properties of Gliese 581 c made it a likely target for future observation missions such as ESA's Darwin Mission and NASA's Terrestrial Planet Finder.[8][34]

See also:

 Sunrise from the Surface of Gliese 581c 
..
Illustration Credit & Copyright: Karen Wehrstein

Explanation: How might a sunrise appear on Gliese 581c? One artistic guess is shown above. Gliese 581c is the most Earth-like planet yet discovered and lies a mere 20 light-years distant. The central red dwarf is small and redder than our Sun but one of the orbiting planets has recently been discovered to be in the habitable zone where liquid water could exist on its surface. Although this planet is much different from Earth, orbiting much closer than Mercury and containing five times the mass of Earth, it is now a candidate to hold not only oceans but life enabled by the oceans. Were future observations to confirm liquid water, Gliese 581c might become a worthy destination or way station for future interstellar travelers from Earth. Drawn above in the hypothetical, the red dwarf star Gliese 581 rises through clouds above a calm ocean of its planet Gliese 581c. - Source: NASA APOD

References:

  1. Udry et al. (2007). "The HARPS search for southern extra-solar planets, XI. Super-Earths (5 and 8 M) in a 3-planet system". Astronomy and Astrophysics 469 (3): L43–L47. doi:10.1051/0004-6361:20077612
  2. Beust, H. et al. (2008). "Dynamical evolution of the Gliese 581 planetary system". Astronomy and Astrophysics 479 (1): 277–282. doi:10.1051/0004-6361:20078794
  3. Than, Ker (2007-04-24). "Major Discovery: New Planet Could Harbor Water and Life". space.com.
  4. Catalog of Nearby Exoplanets—Planets Table". Catalog of Nearby Exoplanets—Planets Table. Exoplanets.org
  5. Than, Ker (2007-02-24). "Planet Hunters Edge Closer to Their Holy Grail". space.com
  6. Selsis et al. (2007). "Habitable planets around the star Gl 581?". Astronomy and Astrophysics 476 (3): 1373–1387. doi:10.1051/0004-6361:20078091
  7. von Bloh et al. (2007). "The Habitability of Super-Earths in Gliese 581". Astronomy and Astrophysics 476 (3): 1365–1371. doi:10.1051/0004-6361:20077939
  8. "New 'super-Earth' found in space". BBC News. April 25, 2007
  9. van Leeuwen, F. (2007). "HIP 74995". Hipparcos, the New Reduction
  10. "The 100 Nearest Stars". RECONS
  11. "Boring Star May Mean Livelier Planet". Spaceref.com
  12. This is obtained by dividing the m sin i of 5.073, by "Sin(10/180 * PI)" on a radian-configured processor.
  13. "Astronomers Find First Earth-like Planet in Habitable Zone". ESO
  14. Valencia et al. (2006). "Radius and Structure Models of the First Super-Earth Planet". The Astrophysical Journal 656 (1): 545–551. doi:10.1086/509800
  15. Valencia and Sasselov (2007). "Detailed Models of Super-Earths: How Well Can We Infer Bulk Properties?". The Astrophysical Journal 665 (2): 1413–1420. doi:10.1086/519554
  16. Seager (2008). "Alien Earths from A to Z". Sky & Telescope ISSN 0037-6604 (January): 22–25
  17. Overbye, Dennis (2007–04–25). "20 light years away, the most Earthlike planet yet". International Herald Tribune
  18. The Earth Worldbook". NASA
  19. Girardi L., Bressan A., Bertelli G., Chiosi C. (2000). "Evolutionary tracks and isochrones for low- and intermediate-mass stars: From 0.15 to 7 M, and from Z=0.0004 to 0.03". Astron. Astrophys. Suppl. Ser. 141: 371. doi:10.1051/aas:2000126.
  20. Vergano, Dan (2007–04–25). "Out of our world: Earthlike planet". USA Today
  21. Selsis 2.4.1 "becomes tidally locked in less than 1 Gyr."
  22. Hut, P. (1981). "Tidal Evolution in Close Binary Systems". Astronomy and Astrophysics 99 (1): 126–140
  23. Perlman, David (2007-04-24). "New planet found: It might hold life". San Francisco Chronicle
  24. Jackson, Brian; Richard Greenberg, Rory Barnes (2008). "Tidal Heating of Extra-Solar Planets". ApJ 681: 1631. doi:10.1086/587641. arΧiv:0803.0026
  25. 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. doi:10.1086/510527
  26. Venus Fact Sheet". NASA
  27. Selsis 5. "Gl 581c is very unlikely to be habitable"
  28. Selsis 3.1 "would be habitable only if clouds with the highest reflectivity covered most of the daytime hemisphere. "
  29. Selsis 3.1.2
  30. Selsis Abstract, 3. Figure 4.
  31. Alpert, Mark (2005-11-07). "Red Star Rising". Scientific American. 
  32. Ralph D Lorenz, Jonathan I Lunine, Paul G Withers, Christopher P. McKay (2001). "Titan, Mars and Earth: Entropy Production by Latitudinal Heat Transport" (PDF). Ames Research Center, University of Arizona Lunar and Planetary Laboratory
  33. "Earth-like planet found that may support life" CTV News
  34. Dennis Overbye (April 25, 2007) "New Planet Could Be Earthlike, Scientists Say". New York Times
Further reading:
Red dwarf star Gliese 581
as seen from the surface of extrasolar planet Gliese 581 c 
..
Illustration Copyright © Walter Myers. All rights reserved.
There is special interest in Gliese 581 c because it is the only known extrasolar planet where liquid water--a necessary ingredient for life as we know it--could exist. Surface temperatures are believed to range between the freezing point of water to about 100° F. However, there are other factors that could affect these values, including the possibility that Gliese 581 c always keeps the same side facing its host star, with the result that one side would become extremely hot while the other extremely cold.

In this image from the surface of Gliese 581 c, its red dwarf host hangs low in the sky over a rocky and watery terrain. This sun has a diameter and radius about a third that of the earth's sun's and it is only about 1/100th as bright. It appears large in the sky because Gliese 581 c orbits relatively close to this red dwarf, completing an orbit in only 14 days.
Credit: Walter Myers

News media reports:

Non-news media SOURCE: Wikipedia Gliese_581c
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