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New data suggest the confirmation of the exoplanet Gliese 581g and the best candidate so far of a potential habitable exoplanet. The nearby star Gliese 581 is well known for having four planets with the outermost planet, Gliese 581d, already suspected habitable. This will be the first time evidence for any two potential habitable exoplanets orbiting the same star. Gliese 581g will be included, together with Gliese 667Cc, Kepler-22b, HD85512, and Gliese 581d, in the Habitable Exoplanets Catalog of the PHL @ UPR Arecibo as the best five objects of interest for Earth-like exoplanets.

Doubts about the existence of Gliese 581g appeared only two weeks after its announcement on September 29, 2010 by astronomers of the Lick-Carnegie Exoplanet Survey. Scientists from the HARPS Team from the Geneva Observatory, which discovered all the previously known four planets around Gliese 581, were not able to detect Gliese 581g out of their own data, which included additional observations.

Further analysis by others scientists also questioned the existence of Gliese 581g in the last two years. Enlarge Comparison of the estimated relative size and orbits of the five exoplanets around Gliese 581. The green shade represent the size of the habitable zone, or the orbital region where an Earth-size planet could have surface liquid water. Planets e, b, and c are too hot for liquid water and life but g and d are in the habitable zone.

Planet g is specially in the right spot for Earth-like conditions while d is marginally within these limits, and colder. This is the first case of a stellar system with two potential habitable exoplanets orbiting the same star. Now the original discoverers of Gliese 581g, led by Steven S. Vogt of UC Santa Cruz, present a new analysis with an extended dataset from the HARPS instrument that shows more promising evidence for its existence.

The new analysis supports their original assumption that all the planets around Gliese 581 are in circular and not elliptical orbits as currently believed. It is under this likely assumption that the Gliese 581g signal appears in the new data. “This signal has a False Alarm Probability of < 4% and is consistent with a planet of minimum mass 2.2M [Earth masses], orbiting squarely in the star’s Habitable Zone at 0.13 AU, where liquid water on planetary surfaces is a distinct possibility” said Vogt.

Based on the new data Gliese 581g probably has a radius not larger than 1.5 times Earth radii. It receives about the same light flux as Earth does from the Sun due to its closer orbital position around a dim red dwarf star. These factors combine to make Gliese 581g the most Earth-like planet known with an Earth Similarity Index, a measure of Earth-likeness from zero to one, of 0.92 and higher than the previously top candidate Gliese 667Cc, discovered last year.

“The controversy around Gliese 581g will continue and we decided to include it to our main catalog based on the new significant evidence presented, and until more is known about the architecture of this interesting stellar system” said Abel Méndez, Director of the PHL @ UPR Arecibo.

Authors on the original paper are Steven S. Vogt, UCO/Lick Observatory, UCSC; Paul Butler, Department of Terrestrial Magnetism, Carnegie Institution; and Nader Haghighipour of the Institute for Astronomy and NASA Astrobiology Institute. Their research is published online on July 20, 2012 in the journal Astronomical Notes, 333, No. 7, 561-575.

For more information: arxiv.org/abs/1207.4515 phl.upr.edu/projects/habitable-exoplanets-catalog

The Daily Galaxy via Planetary Habitability Laboratory


Unfortunately the Terrestial Planet Finder is not going ahead for now and the James Webb space telescope is going ahead but I don't know if it can discern continents and oceans. Is there any other existing or soon existing telescope that will be able to detect oceans and continents on terrestrial extrasolar planets and determine if Gliese 581 g (or d) of some of the other habitable planets around red dwarfs are tidally locked?

Boy this sure is exciting! Hope we can one day get more people fascinated by the wonder of space!

If anyone is interested, a group of filmmakers are looking to make a documentary that will help enlighten more people on our current situation as far as space travel goes, so that perhaps sooner than later we can get things back in the right direction. Sorry, not trying to spam or anything here, just seemed like a good place to mention it. Thanks guys.


If they are habitable, it would be insane to walk on another world 20 light years from home. if there was a high speed internet relay going to said planet, it would take probably 20 years and 5 seconds to download an mp3 file. It is just so far, I think we can't live there.

Hi Bran;
I think you are too dependent on the internet. I'm sure there are still some more pioneering people that would be willing to venture out into space to start a new civilization on another planet that could be autonomous from earth. To make sure that people could be self-sustaing on Gliese 581 g a robot probe could be sent there or on a flyby first or as an orbiter or even lander. If you are so dependent on earth for going 20 light years away how do you think humanity could set up bases on more remote parts of the galaxy. Would you also have been reluctant to carry out the Apollo moon landings because there was then no internet or too much moon orbiter coordination?

I'm pretty sure that Bran was saying that the planet is too far
for us to make the travel. He was comparing our internet download
speed to our speed of travel, since it's faster.
The planet may be inhabitable, but we wouldn't survive the distance.

I'd love to see if we could though--decades from now.
It makes me wonder how many different elements are only
found in other galaxies.

Hi Landon;
We might not be able to make it in one human lifetime now. However I think with nucler fission or later nucler fusion powered rockets people could do that within a couple generations. It would probably take about 100 years one way as I heard nucler powered rockets could get up to about 1/5 the speed of light. Nuclear fission rockets could be built now if there was the political, financial and social support. Testing was done in them in the 1960s that was successful but they were stopped because of the nucler test ban treaty in 1972. A nuclear rocket or solar sail for an unmanned trip could probably be built even sooner as it could be less massive and is even more within current technology. The Japanese were quite successful in launching and operating a solar sail they sent towards the planet Venus. It is important to have vision for long range space exploration plans but it would be also important to have political or business and financial and some social backing.

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