GILESE 581
HABITABLE PLANET
Ankur Kumar
INTRODUCTION
Gliese 581 is a star of spectral type M3V (a red dwarf) at the center of the Gliese 581 planetary
system, about 20 light years away from Earth in the Libra constellation. Its estimated mass is
about a third of that of the Sun, and it is the 89th closest known star to the Sun. Gliese 581 is
known at least from 1886, when it was included in Southern Durchmusterung (SD)the fourth
part of the Bonner Durchmusterung. The corresponding designation is BD -7 4003.
The name Gliese 581 refers to the catalog number from the 1957 survey Gliese
Catalogue of Nearby Stars of 965 stars located within 20 parsecs of the Earth. 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. The star is a red dwarf with spectral type M3V, located
20.4 light-years away from Earth. It is located about two degrees north of Beta
Librae, the brightest star in the Libra constellation. Its mass is estimated to be
approximately a third that of the Sun, and it is the 89th closest known star system to
the Sun.
ASPECTS
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 fuse hydrogen at a significantly lower rate. From the apparent
magnitude and distance, astronomers have estimated an effective temperature of 3200 K
and a visual luminosity of 0.2 percent of that of the Sun. However, a red dwarf such as Gliese
581 radiates primarily in the near infrared, with peak emission at a wavelength of roughly
830 nm (estimated using Wien's displacement law, which assumes the star radiates as
a black body), so such an estimate will underestimate the star's total luminosity.[5] (For
comparison, the peak emission of the Sun is roughly 530 nm, in the middle of the visible
part of the spectrum). When radiation over the entire spectrum is taken into account (not
just the part that humans are able to see), something known as the bolometric correction,
this star has a bolometric luminosity 1.3% of the Sun's total luminosity. 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 prosaically,
the habitable zone. The extent of such a zone is not fixed and is highly specific for
each planetary system.
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Gliese 581 is classified as a variable star of the BY Draconis type, and has been
given the variable star designation HO Librae. This is a star that exhibits variability
because of the presence of star spots combined with the rotation of the star.
However, the measured variability is close to the margin of error, and, if real, is
most likely a long term variability. Its brightness is stable to 1%. Gliese 581 emits X-
rays.
CONSTELLATION LIBRA B-V COLOR 1.61 ABSOLUTE 11.6
INDEX MAGNITUDE
RIGHT 15h 19m 26.8250 RADIAL 9.5 0.5 MASS 0.31
ASCENSION VELOCITY
DECLINATION 07 43 20.209 PROPER RADIUS 0.29
RA: 1233.51
MOTION
APPARENT 10.56 to 10.58 PARALLAX 160.12 1.33 LUMINOSITY 0.013
MAGNITUDE
SPECTRAL TYPE M3V DISTANCE 20.4 0.2 ly SURFACE 4.920.10
GRAVITY
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The Gliese 581 planetary system[24]
Companion Semimajor Orbital
Eccentric Inclinati Radi
(in order from Mass axis period
star)
ity on us
(AU) (days)
1.7 0.2 0.02815 0.00 3.1490 0.0
e 0.00-0.06
M 006 002
15.8 0.3 0.04061 0.00 5.3686 0.0
b 0.00-0.03
M 003 001
5.5 0.3 0.0721 0.000 12.914 0.00
c 0.00-0.06
M 3 2
g (unconfirmed
2.2 M 0.13 32 0.00
)
d[25] (unconfir 6.98 0.3 0.21847 0.00
66.87 0.13 0.00-0.25
med) M 028
Debris
25 12 AU>60 AU 30 70
disk[26]
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HABITABILITY
The habitability is determined by a large number of factors from a variety of sources.
Although the low stellar flux, high probability of tidal locking, small circumstellar
habitable zones, and high stellar variation experienced by planets of red dwarf stars
are impediments to their planetary habitability, the ubiquity and longevity of red
dwarfs are positive factors. Determining how the interactions between these factors
affect habitability may help to reveal the frequency of extraterrestrial life and
intelligence.
Intense tidal heating caused by the proximity of planets to their host red dwarfs is a
major impediment to life developing in these systems. Other tidal effects, such as
the extreme temperature differences created by one side of habitable-zone planets
permanently facing the star and the other perpetually turned away and lack of
planetary axial tilts, reduce the probability of life around red dwarfs. Non-tidal
factors, such as extreme stellar variation, spectral energy distributions shifted to
the infrared relative to the Sun, and small circumstellar habitable zones due to low
light output, further reduce the prospects for life in red-dwarf systems.
There are, however, several effects that increase the likelihood of life on red dwarf
planets. Intense cloud formation on the star-facing side of a tidally locked planet
may reduce overall thermal flux and drastically reduce equilibrium
temperature differences between the two sides of the planet. In addition, the sheer
number of red dwarfs, which account for about 85% of at least 100 billion stars in
the Milky Way, statistically increases the probability that there might exist habitable
planets orbiting some of them. As of 2013, there are expected to be tens of billions
of super-Earth planets in the habitable zones of red dwarf stars in the Milky Way.
LUMINOSITY AND SPECTRAL COMPOSITION
For years, astronomers ruled out red dwarfs, with masses ranging from roughly 0.08
to 0.45 solar masses (M), as potential abodes for life. The low masses of the stars
cause the nuclear fusion reactions at their cores to proceed exceedingly slowly,
giving them luminosities ranging from a maximum of roughly 3 percent that of the
Sun to a minimum of just 0.01 percent. Consequently, any planet orbiting a red dwarf
would have to have a low semimajor axis in order to maintain Earth-like surface
temperature, from 0.3 astronomical units (AU) for a relatively luminous red dwarf
like Lacaille 8760 to 0.032 AU for a smaller star like Proxima Centauri, the nearest
star to the Solar System. Such a world would have a year lasting just six days.
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Much of the low luminosity of a red dwarf falls in the infrared part of the
electromagnetic spectrum, with lower energy than the visible light in which the Sun
peaks. As a result, photosynthesis on a red dwarf planet would require additional
photons to achieve excitation potentials comparable to those needed in Earth
photosynthesis for electron transfers, due to the lower average energy level of near-
infrared photons compared to visible. Having to adapt to a far wider spectrum to
gain the maximum amount of energy, foliage on a habitable red dwarf planet would
probably appear black if viewed in visible light.
In addition, because water strongly absorbs red and infrared light, less energy would
be available for aquatic life on red dwarf planets. However, a similar effect of
preferential absorption by water ice would increase its temperature relative to an
equivalent amount of radiation from a Sun-like star, thereby extending the habitable
zone of red dwarfs outward.
Another fact that would inhibit habitability is the evolution of the Red Dwarf stars;
as such stars have an extended pre-main sequence phase, their eventual habitable
zones would be for around 1 billion years a zone where water wasn't liquid but in its
gaseous state. Thus, terrestrial planets in the actual habitable zones, if provided with
abundant surface water in their formation, would have been subject to a runaway
greenhouse effect for several hundred million years. During such an early runaway
phase, photolysis of water vapor would allow hydrogen escape to space and the loss
of several Earth oceans of water, leaving a thick abiotic oxygen atmosphere.
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