Does 'Goldilocks' Planet Gliese 581g Harbor Life?
by Rich Deem


Scientists recently announced the discovery of an "earth-like" planet revolving around the dwarf red star Gliese 581a. According to their calculations, this planet should be at the right distance from its star to contain liquid water.1 Does this mean that there is a good probability for the existence of life?

Rich Deem


Scientists have discovered over 500 planets outside our own solar system. However, most of these planets are quite unlike the earth, being much larger and much closer to their parent star. At least part of the reason for the difference is due to the ease of detection of larger planets, since they gravitationally disturb their parent star much more than smaller planets. The recent proclamation by astronomer Steven Vogt that "the chances of life on this planet are 100 percent" has the news media buzzing. However, there are some big assumptions that went into this claim, many of which are certainly false.

Parent star

Stars and planets are given names according to the following convention. A star's name usually includes some numbers identifying it. For example, the parent star of the newly discovered planet is called Gliese 581 or Gliese 581a, with the "a" designating the star. Subsequent stars and/or planets are given the next letter, in order of discovery. For example, in a binary system (a system containing two stars orbiting each other), the second star would be given the letter "b". If the planetary system has only one star, the first planet discovered will be given the designation "b". So, we know from the name, Gliese 581g, that it is the sixth discovered planet around the star Gliese 581a. A table containing the characteristics of each of the planets of the Gliese 581a system are shown below:

Orbital Parameters for GJ 581 Planet Candidates
Planet Period (days) K
m sin i (M) a (AU) Mean Anomalya (°) FAPS
b 5.36841 (0.00026) 12.45 (0.21) 15.6 (0.3) 0.0406163 (1.3e-6) 276.1 (4.9) 6.8E-306
c 12.9191 (0.0058) 3.30 (0.19) 5.6 (0.3) 0.072993 (2.2e-5) 33 (19) 2.3E-33
d 66.87 (0.13) 1.91 (0.22) 5.6 (0.6) 0.21847 (2.8e-4) 56 (27) 0.0000025
e 3.14867 (0.00039) 1.66 (0.19) 1.7 (0.2) 0.0284533 (2.3e-6) 267 (40) 1.9E-08
f 433 (13) 1.30 (0.22) 7.0 (1.2) 0.758 (0.015) 118 (68) 0.000095
g 36.562 (0.052) 1.29 (0.19) 3.1 (0.4) 0.14601 (1.4e-4) 271 (48) 0.0000027

Solar emission

The star itself is rather typical of stars in our galaxy, being a red dwarf, about one third the mass of our star the Sun. The fact that the star is much smaller than our star has some profound implications regarding possible life on the planet. First, the surface temperature of the star is less than a third that of the Sun.2 This means that the habitable (or Goldilocks) zone is much closer to the star. The small mass and lower temperature means that the star is emitting energy primarily in the longer wavelengths (lower energy). This is why the star appears red (mean radiance at 830 nm), instead of yellow, like the Sun (mean radiance at 530 nm). Light emitted primarily at the red end of the visible spectrum will profoundly affect possible photosynthesis on nearby planets. Photosynthetic pigments of organisms on earth absorb energy in the higher energy wavelengths associated with visible light, that is, violet or blue, and to a lesser extent, red. However, if you try to grow your houseplant in red light it will promptly die.

Although chlorophylls absorb energy down to 700 nm, a dwarf star with mean radiance at 830 nm will find most of its emission unusable by photosynthetic organisms. Is it possible that organisms that evolve around red dwarfs will develop photosynthetic pigments that absorb energy from the red/infrared end of the visible spectrum? Although it is possible, the physics of electromagnetic radiation severely limits the efficiency of such a pigment, since the reduced energy of such light might be insufficient to excite electrons to a higher energy state (during the light reaction phase of photosynthesis). In addition to the photosynthesis problem, red dwarfs suffer from frequent solar flares, which would periodically dowse the planet with high levels of radiation.


By solar standards, Gliese 581a is relatively "metal-poor." In astronomical terms, "metal" does not necessarily refer to things like iron, but to any elements heavier than helium. Initially, during the beginning of the Big Bang, only hydrogen was formed. However, there was a period of time (about three minutes) in which the temperature of the universe was high enough to support nuclear fusion. It was during this time that hydrogen was fused to form helium, which made up about 25% of the universe. The heavier elements that make up rocky planets were formed in the cores of generations of stars preceding the current generation. Large stars use their fuel quickly and end catastrophically as a supernovae. The metals produced by these stars are then incorporated into the next generation of planetary systems. Since Gliese 581a is metal-poor compared with the Sun, it is surprising it has at least 5 rocky planets. However, because of its metallicity, it is likely that those planets are less dense than their solar system equivalents.

Planet Gliese 581g

Planet mass

Although declared to be "earth-like," Gliese 581g is actually at least 3 times more massive than earth. This increased mass likely impacts the atmospheric content of the planet. Planets must be of a certain mass in order to retain an atmosphere. For example, Mars has only 10% the mass of the earth. Extensive evidence indicates that it once had a thicker atmosphere and large bodies of water on its surface. However, because of its reduced gravity, it lost nearly all of that atmosphere and water. The earth originally had a much denser atmosphere that was mostly blasted into space during the collision that formed the Moon. Had it not been for this fortuitous collision, the earth would have had an atmosphere similar to Venus (81% the mass of the earth), which has an atmosphere 80 times denser than earth. One of the assumptions of the paper that reports the discovery of Gliese 581g is that the atmosphere is similar to earth's. This assumption is almost certainly wrong, unless the planet experienced a large collision event, similar to earth's, early in its history. With at least 3 times the mass of earth, the planet would likely have an atmosphere much denser than Venus, making life there extremely unlikely. The problem with such a dense atmosphere is the large greenhouse effect, which would raise temperatures dramatically. The fact that Gliese 581a emits light mostly in the red/infrared end of the spectrum would compound the heating effect.

Tidal locking

The Privileged PlanetBecause Gliese 581g orbits close to its star (0.15 AU) and the system is at least 7 billion years old, the planet is tidally locked with its star. This means that the planet rotates once every 37 days, the same period over which it revolves around its star. So, one side of Gliese 581g is perpetually in starlight and the other in perpetual darkness. This fact results in severe conditions over the majority of the planet's surface. Assuming an atmosphere similar to earth's, the sunlit side has an average temperature of 160°F, while the dark side has an average temperature of -25°F. Only a small strip of the planet, near the edges between sunlight and darkness would exist at a reasonable temperature for life to exist. However, in these twilight areas, the amount of solar irradiance would be very low, compounding the mean radiance wavelength problem, making photosynthesis all but impossible. The location of the twilight areas of Gliese 581g will not remain stable over time, since all planets tend to wobble around their axis of rotation, causing these "life zones" to migrate over time.

Does Gliese 581g really exist?

Another astronomy team at Geneva Observatory attempted to plug their data on Gliese 581 into computer models and did not see the planet at all, over the random noise seen in the data:

"Simulations on the real data have shown that the probability that such a signal is just produced 'by chance' out of the noise is not negligible, of the order of several percents. Under these conditions we cannot confirm the presence of the announced planet Gliese 581 g."3

Conclusion Top of page

The newly discovered planet Gliese 581g has been advertised as being within the "Goldilocks" zone around its star, leading to the claim, "the chances of life on this planet are 100 percent." The reality of the discovery is that the star is a red dwarf, being much dimmer than our Sun. The nearness of the planet to its star insures that the planet is tidally locked with temperatures on the sunny side being 160°F, while the average temperature on the dark side being -25°F. Only small portions of the planet in the twilight zones are at temperatures conducive to the existence of life. However, the emission spectrum of the star averages in the infrared wavelengths, making photosynthesis problematic. The higher mass of the planet virtually guarantees that its atmospheric pressure is quite high, likely resulting in a runaway greenhouse effect. Rather than being a prime candidate for life, Gliese 581g is likely a hot, sterile world devoid of water. Further investigation of the planet is likely, including possible atmospheric spectra, which will reveal the truth about Gliese 581g, if such a planet really exists at all. One must ask whether the odds of life would remain at 100% if the planet is really just noise in the data. Stay tuned, but don't get in your rocket ship to go for a visit!

Rare EarthRare Earth: Why Complex Life is Uncommon in the Universe by Peter D. Ward and Donald Brownlee

A secular book that recognizes the improbable design of the earth. Paleontologist Peter D. Ward and astrobiologist Donald Brownlee examine the unusual characteristics of our galaxy, solar system, star, and Earth and conclude that ET may have no home to go to. Surprisingly, the authors conclude that the amazing "coincidences" are the result of good luck and chance.

The Creator and the CosmosThe Creator and the Cosmos by Dr. Hugh Ross

A classic book for modern Christian apologetics and science, recently updated with fully one third of the book updated. Dr. Ross presents the latest scientific evidence for intelligent design of our world and an easy to understand introduction to modern cosmology. This is a great book to give agnostics, who have an interest in cosmology and astronomy.

References Top of page

  1. Vogt, S. S., R. P. Butler, E. J. Rivera, N. Haghighipour, G. W. Henry, and M. H. Williamson. 2010. The Lick-Carnegie Exoplanet Survey: A 3.1M Planet in the Habitable Zone of the Nearby M3V Star Gliese 581. Submitted for publication.
  2. Gliese 581, Wikipedia.
  3. Leslie Mullen. 2010. Doubt cast on existence of possibly habitable alien planet, Astrobiology Magazine.

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