Gliese 581 d

Gliese 581 d

Extrasolar planet		List of extrasolar planets
An artist's impression of Gliese 581 d and speculative moons.
Parent star
Star		Gliese 581
Constellation		Libra
Right ascension	(α)	15h 19m 26s
Declination	(δ)	−07° 43′ 20″
Apparent magnitude	(mV)	10.5
Distance		20.3 ± 0.3 ly (6.2 ± 0.1 pc)
Spectral type		M3P00P00
Mass	(m)	0.31 M☉
Radius	(r)	0.29 R☉
Temperature	(T)	3480 ± 48 K
Metallicity	[Fe/H]	-0.33 ± 0.12
Age		7 – 11 Gyr
Orbital elements Epoch JD 2451409.762
Semimajor axis	(a)	0.21847 ± 0.00028 AU
Eccentricity	(e)	0
Orbital period	(P)	66.87 ± 0.13 d (200.183 y)
		(1600 h)
Mean anomaly	(M)	56 ± 27°
Semi-amplitude	(K)	1.91 ± 0.22 m/s
Physical characteristics
Minimum mass	(m sin i)	5.6 ± 0.6 M⊕
Discovery information
Discovery date		24 April 2007
Discoverer(s)		Udry et al.
Detection method		Radial velocity
Discovery site		La Silla Observatory, Chile
Discovery status		Published
Other designations
HO Librae d, HO Lib d, BD−07°4003 d, GJ 581 d, HIP 74995 d, LFT 1195 d, LHS 394 d, LPM 564 d, LTT 6112 d, NLTT 39886 d, TYC 5594-1093-1 d, Wolf 562 d.
Database references
Extrasolar Planets Encyclopaedia		data
SIMBAD		data

Gliese 581 d ( /ˈɡliːzə/) or Gl 581 d is an extrasolar planet orbiting the star Gliese 581 approximately 20 light-years away in the constellation of Libra. It is the third planet discovered in the system and the fifth in order from the star.

Because of its mass, at least 5.6 times that of Earth, the planet is classified as a super-Earth. Originally believed to be outside the habitable zone, in late April 2009 new observations made by the original discovery team concluded that the planet is on the outskirts of the habitable zone where liquid water may exist. In May 2011 researchers in France released a study of a three-dimensional climate simulation concluding that it is plausible that the planet has a stable atmosphere and liquid water on the surface, concurring that it is the first discovered terrestrial-mass exoplanet in the habitable zone, followed by HD 85512 b several months later.

Discovery

A team of astronomers led by Stéphane Udry of the Geneva Observatory used the HARPS instrument on the European Southern Observatory 3.6 meter telescope in La Silla, Chile to discover the planet in 2007. Udry's team employed the radial velocity technique, in which the mass of a planet is determined based on the small perturbations it induces in its parent star’s orbit via gravity.

Physical characteristics

The motion of the parent star indicates a minimum mass for Gliese 581 d of 5.6 Earth masses (earlier analyses gave higher values). Dynamical simulations of the Gliese 581 system assuming that the orbits of the three planets are coplanar show that the system becomes unstable if the masses of the planets exceed 1.6 – 2 times the minimum values. Using earlier minimum mass values for Gliese 581 d, this implies an upper mass limit for Gliese 581 d of 13.8 Earth masses.

Climate and habitability

It was originally thought that Gliese 581 d orbits outside the habitable zone of its star. However, in 2009 the original discovery team revised its original estimate of the planet's orbital parameters, finding that it orbits closer to its star than originally believed. They concluded that the planet is within the habitable zone where liquid water could exist, corroborated in 2011 by a climate simulation study. According to Stéphane Udry, "It could be covered by a 'large and deep ocean'; it is the first serious ocean planet candidate."

On average, the light that Gliese 581 d receives from its star has about 30% of the intensity of sunlight on Earth. By comparison, sunlight on Mars has about 40% of the intensity of that on Earth. That might seem to suggest that Gliese 581 d is too cold to support liquid water and hence is inhospitable to life. However, an atmospheric greenhouse effect can significantly raise planetary temperatures. For example, Earth's own mean temperature would be about −18°C without any greenhouse gases, ranging from around 100°C on the day side to −150°C at night, much like that found on the moon. If the atmosphere of Gliese 581 d produces a sufficiently large greenhouse effect, and the planet's geophysics stabilize the CO₂ levels (as Earth's does via plate tectonics), then the surface temperature might permit a liquid water cycle, conceivably allowing the planet to support life. Calculations by Barnes et al. suggest, however, that tidal heating is too low to keep plate tectonics active on the planet, unless radiogenic heating is somewhat higher than expected. In recent years, discoveries have led scientists to believe that Gliese 581 d may be tidally locked, which means that it has permanent day and night side. If however, Gliese 581 has four planets in orbit, instead of six, it is expected to be in an eccentric orbit and slowly rotating pseudo-synchronously, in a similar fashion to the planet Mercury. Gliese 581 d would then approximately rotate twice for each orbit of its parent star.

Gliese 581 d is probably too massive to be made only of rocky material. It may have originally formed on a more distant orbit as an icy planet that then migrated closer to its star.

Messages from Earth

Another artist impression of Gliese 581 d as a Super-Earth.

In October 2008, members of the networking website Bebo beamed A Message From Earth, a high-power transmission at Gliese 581, using the RT-70 radio telescope belonging to the National Space Agency of Ukraine. This transmission is due to arrive in the Gliese 581 system's vicinity by the year 2029; the earliest possible arrival for a response, should there be one, would be in 2049.

As part of the 2009 National Science Week celebrations in Australia, Cosmos Magazine launched a website called Hello From Earth to collect messages for transmission to Gliese 581d. The maximum length of the messages was 160 characters, and they were restricted to the English language. In total, 25,880 messages were collected from 195 countries around the world. The messages were transmitted from the DSS-43 70 m radio telescope at the Canberra Deep Space Communication Complex at Tidbinbilla, Australia on the 28th of August, 2009.

See also

• HD 85512 b
• Kepler-22b
• List of potential habitable exoplanets

External links

• National Science Week 2009 - Send A Message To Gliese 581d
• Wordsworth, R. D. et al (2010). "Is Gliese 581d habitable? Some constraints from radiative-convective climate modeling". Astronomy & Astrophysics 522: A22. arXiv:1005.5098. Bibcode 2010A&A...522A..22W. doi:10.1051/0004-6361/201015053.