By Amir Alexander
April 22, 2009
The hunt for distant Earths received a tremendous boost this week with the announcement of two remarkable discoveries in a planetary system 20.5 light-years away. In an article submitted to the journal Astronomy & Astrophysics, a group of European astronomers reported the detection of a planet estimated to be only twice the mass of Earth. It is by far the smallest world yet discovered orbiting a faraway star. In the same article the authors also report that additional observations of another planet in the same system place it at heart of its star's "habitable zone," where liquid water is stable.
"The holy grail of current exoplanet research is the detection of a rocky, Earth-like planet in the 'habitable zone' said lead author Michel Mayor of the Geneva Observatory. And while neither of the planets discussed in the article fully answers to this description, their discovery suggests that the detection of other Earths may not be far behind.
The new "light weight" planet is the fourth known member of the planetary system around the red dwarf star Gliese 581, and is designated accordingly Gliese 581e or, more technically, GJ 581e. According to Xavier Bonfils of the Grenoble Observatory, who was a member of the discovery team, with a minimum mass of 1.9 Earths the planet is most likely a rocky world like our own. But as the innermost planet in the system it is extremely close to its star, completing each orbit in just over 3 days. Its surface is therefore far too hot to sustain liquid water or life as we know it.
This however is not the case for its larger neighbor, the planet Gliese 581d, whose minimum mass is seven times that of the Earth. Previous estimates had the planet orbiting its star every 82 days, a period that placed it just beyond the outer boundary of the habitable zone. Additional data gathered over the past two years, however, has disclosed that the planet's true orbital period is only 67 days, placing it closer to its star and right at the heart of the habitable zone.
"Gliese 581d is probably too massive to be made only of rocky material," said discovery team member Stephane Udry of the Geneva observatory, "but we can speculate that it is an icy planet that has migrated closer to the star." Since it is located in the habitable zone, Udry continued, it "could even be covered by a large and deep ocean – it is the first serious 'water world' candidate."
The two planets, Gliese 581d and Gliese 581e, were detected and observed with the HARPS spectrograph, at the European Southern Observatory's 3.6 meter telescope at La Silla, Chile. Built specifically to search for exoplanets through the radial velocity method, HARPS can detect the miniscule shifts in a star's spectrum as it rocks back and forth to the tug of an orbiting planet. HARPS is so sensitive, in fact, that it can register a star's motion towards the Earth or away from it, even if it is as slow as 1 meter per second -- about the speed of a leisurely stroll. It is thanks to this remarkable sensitivity that the spectrograph was able to discover a planet as small as Gliese 581e.
In the future, radial velocity searches may become even more sensitive. With funding from The Planetary Society, planet hunter Geoff Marcy of U.C. Berkeley is building a system known as FINDS Exo-Earths, which combines two advanced optical technologies to enhance the light signal from observed stars and improve the "signal to noise" ratio. When in the coming years the system will be installed at the Keck Observatory in Hawaii, Marcy predicts it will be able to detect stellar motions as slow as 0.5 meters per second. With that kind of sensitivity, the detection of Earth-mass planets will be within reach.
The radial velocity method employed by HARPS has been the most effective planet-hunting tool to date, but it does have this peculiarity: while it can provide an estimate of a planet's minimum mass, it cannot directly measure a planet's true mass. This depends on the inclination of the planet's orbital plane as seen from Earth, which in most cases is not known. And so, while the planetary masses measured by this method may be low, it is nevertheless theoretically possible that the true masses of the planets are, in fact much higher.
Significantly, however, this is not the case for the unique planetary system around Gliese 581. By running repeated computer simulations, Mayor and his collaborators showed that the true masses of the planets orbiting Gliese 581 could not be greater than 1.6 times their minimum values. If the planets were more massive than that, the simulations showed, the system would be inherently unstable and would fall apart within a few million years. This means that at the most, Gliese 581e cannot be more – and is likely considerably less – than 3 Earth masses.
"It is amazing to see how far we've come" said Mayor, who in 1995 led the team that discovered the first exoplanet around a normal star – a gas giant orbiting 51 Pegasi. "The mass of Gliese 581e is 80 times less than that of 51 Pegasi b" he said, reflecting on the evolution of the hunt for exoplanets. "This is tremendous progress in just 14 years."
Indeed it is. And as new and ever more sensitive technologies come online, the day when an Earth-mass rocky planet will be found orbiting in the habitable zone of a distant star is fast approaching.
Vedic Perspective
Srimad Bhagavatam 1.14.35-36 P The Disappearance of Lord Krsna
TEXTS 35-36
TEXT
mangalaya ca lokanam
ksemaya ca bhavaya ca
aste yadu-kulambhodhav
adyo 'nanta-sakhah puman
yad bahu-danda-guptayam
sva-puryam yadavo 'rcitah
kridanti paramanandam
maha-paurusika iva
SYNONYMS
mangalaya--for all good; ca--also; lokanam--of all the planets; ksemaya--for protection; ca--and; bhavaya--for elevation; ca--also; aste--is there; yadu-kula-ambhodhau--in the ocean of the Yadu dynasty; adyah--the original; ananta-sakhah--in the company of Ananta (Balarama); puman--the supreme enjoyer; yat--whose; bahu-danda-guptayam--being protected by His arms; sva-puryam--in His own city; yadavah--the members of the Yadu family; arcitah--as they deserve; kridanti--are relishing; parama-anandam--transcendental pleasure; maha-paurusikah--the residents of the spiritual sky; iva--like.
TRANSLATION
The original Personality of Godhead, the enjoyer, and Balarama, the primeval Lord Ananta, are staying in the ocean of the Yadu dynasty for the welfare, protection and general progress of the entire universe. And the members of the Yadu dynasty, being protected by the arms of the Lord, are enjoying life like the residents of the spiritual sky.
PURPORT
As we have discussed many times, the Personality of Godhead Visnu resides within each and every universe in two capacities, namely as the Garbhodakasayi Visnu and the Ksirodakasayi Visnu. The Ksirodakasayi Visnu has His own planet on the northern top of the universe, and there is a great ocean of milk where the Lord resides on the bed of the Ananta incarnation of Baladeva. Thus Maharaja Yudhisthira has compared the Yadu dynasty to the ocean of milk and Sri Balarama to the Ananta where Lord Krsna resides. He has compared the citizens of Dvaraka to the liberated inhabitants of the Vaikunthalokas. Beyond the material sky, further than we can see with our eyes and beyond the sevenfold coverings of the universe, there is the Causal Ocean in which all the universes are floating like footballs, and beyond the Causal Ocean there is an unlimited span of spiritual sky generally known as the effulgence of Brahman. Within this effulgence there are innumerable spiritual planets, and they are known as the Vaikuntha planets. Each and every Vaikuntha planet is many, many times bigger than the biggest universe within the material world, and in each of them there are innumerable inhabitants who look exactly like Lord Visnu. These inhabitants are known as the Maha-paurusikas, or persons directly engaged in the service of the Lord. They are happy in those planets and are without any kind of misery, and they live perpetually in full youthfulness, enjoying life in full bliss and knowledge without fear of birth, death, old age or disease, and without the influence of kala, eternal time. Maharaja Yudhisthira has compared the inhabitants of Dvaraka to the Maha-paurusikas of Vaikunthaloka because they are so happy with the Lord. In the Bhagavad-gita there are many references to the Vaikunthalokas, and they are mentioned there as mad-dhama, or the kingdom of the Lord.
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