Gliese 581

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＜主星＞Gliese 581g、Gl 581g、GJ 581g

てんびん座

赤経　15h 19m 26s

赤緯　－07° 43′ 20″

視級　10.55

距離　20.3 ± 0.3 ly(6.2 ± 0.1 pc)

オーストラリアにあるウエストシドニー大学のラグバー博士の調査・研究により、遥か銀河の彼方から、地球外知的生命体らしき存在からのパルス信号が発信されている事が明らかとなった。地球外の文明による信号の可能性があるという。

パルス信号が発信されているのは恒星グリーゼ581をまわる惑星からで、地球から20光年離れた恒星グリーゼ581を中心とした太陽系型の天体にあるという。そこには6つの惑星があり、そのひとつは以前から生命体がいてもおかしくないと言われていた惑星グリーゼ581Gであることから、大きな注目を集めている。

この惑星グリーゼ581Gは水と緑に包まれた自然豊かな地球型惑星の可能性もあり、この惑星をイメージして描かれたCGイラストは非常に緑豊かな惑星として描かれている。どのような知的生命体が生活しているのかは不明だが、パルス信号が確かなものであれば地球人と同等か、それ以上の科学力を持っている可能性がある。

ラグバー博士は2年前から恒星グリーゼ581の周囲の調査を行っており、そのときからパルス信号の規則性を調査していた。そして今になり、このパルス信号が人工的な文明によるパルス信号の可能性が高いと判断したようだ。

惑星グリーゼ581Gはパルス信号が注目される以前から、雲や液体の水の存在や、生命が存在していてもおかしくない環境下にある惑星として注目されており、その恒星グリーゼ581の周囲から規則的なパルス信号が発せられているとあれば、グリーゼGに生命が存在しないと思うほうが不自然といえる。

ラグバー博士は恒星グリーゼ581の周囲にある惑星に対して、「私の個人的な意見でしかないが、この惑星に生命が存在する可能性は100パーセントといえるだろう」とコメントしている。しかし、惑星グリーゼ581Gの知的生命体と交信をするにしても、質問を投げかけて返信されてくるのは40年後という事になる。高速の宇宙船で惑星グリーゼ581Gに向かったとしても到着まで20年かかり、現在の地球の科学力ならば何百年かかるかわからない。

もし最速のファーストコンタクトを望むのであれば、こちらの位置を電波で伝え、惑星グリーゼ581Gの知的生命体のワープ航行等の技術で地球に来てもらったほうが早いかもしれない。とにかく、夢が広がるこのニュース。惑星グリーゼ581Gに友好的な知的生命体がいることを願いたい。

ちなみに、「グリーゼ581」とは太陽の役割をしている恒星と6つの惑星を意味する呼び名で、恒星グリーゼ581のほかに惑星グリーゼ581B、C、D、E、F、Gがある。生命体がいると予想されているのは、すでに説明している通り惑星グリーゼ581Gである。

Screenshot: dailymail.

参考URL
http://www.astroarts.co.jp/news/2010/10/01gliese581g/index-j.shtml

http://topics.jp.msn.com/life/column.aspx?articleid=432778

http://keckobservatory.org/

http://rocketnews24.com/?p=52546

http://keckobservatory.org/news/keck_observatory_discovers_the_first_goldilocks_exoplanet/

http://www.ucolick.org/~vogt/

http://carnegiescience.edu/news/potentially_habitable_planet_discovered

http://www.nasa.gov/home/hqnews/2010/sep/HQ_10-237_Exoplanet_Findings.html

http://www.nsf.gov/news/news_images.jsp?cntn_id=117765&org=NSF

http://news.sciencemag.org/sciencenow/2010/09/astronomers-find-most-earth-like.html

動画
http://www.youtube.com/watch?v=ZwNMg5JB_ic&feature=player_embedded#!

http://www.youtube.com/watch?v=bkVHnL7I46Q

http://www.youtube.com/watch?v=nj6ABUFUu_I

http://www.youtube.com/watch?v=K9Rq4ZBx-Jg

http://www.youtube.com/watch?v=wJXSSYyIVqw

September 29, 2010

View a webcast with Steven Vogt and Paul Butler, leaders of a team that discovered the first potentially habitable exoplanet.

A team of planet hunters led by astronomers at the University of California, Santa Cruz (UC Santa Cruz), and the Carnegie Institution of Washington, and supported by the National Science Foundation (NSF) and NASA, has announced the discovery of an Earth-sized planet (three times the mass of Earth) orbiting a nearby star at a distance that places it squarely in the middle of the star's "habitable zone," where liquid water could exist on the planet's surface. If confirmed, this would be the most Earth-like exoplanet yet discovered and the first strong case for a potentially habitable one.

"This is clearly one of the most exciting areas of science these days" said Ed Seidel, assistant director for NSF's Mathematical and Physical Sciences directorate. "If we do discover life outside our planet, it would perhaps be the most significant discovery of all time."

To astronomers, a "potentially habitable" planet is one that could sustain life, not necessarily one that humans would consider a nice place to live. Habitability depends on many factors, but liquid water and an atmosphere are among the most important.

"Our findings offer a very compelling case for a potentially habitable planet," said Steven Vogt, professor of astronomy and astrophysics at UC Santa Cruz. "The fact that we were able to detect this planet so quickly and so nearby tells us that planets like this must be really common."

"With modern techniques, it is now possible to actually search for worlds that might be able to support life as we understand it," added Seidel. "Just a few years back I wouldn't have thought this could have advanced so fast."

This discovery was the result of over a decade of observations on the W. M. Keck Observatory in Hawaii. "Advanced techniques combined with old-fashioned ground-based telescopes continue to lead the exoplanet revolution," said Paul Butler of the Carnegie Institution. "Our ability to find potentially habitable worlds is now limited only by our telescope time."

"One of the three main science objectives of the Astronomy and Astrophysics Decadal Survey released last month is labeled 'New Worlds: Seeking nearby habitable planets,'" added NSF Astronomy Division Director Jim Ulvestad. "It is very gratifying to see that long-term scientific investments by NSF and NASA toward meeting this objective are paying off, and we expect continued discoveries in this area as nearby stars are monitored for longer periods."

Vogt and Butler lead the Lick-Carnegie Exoplanet Survey. The team's new findings are reported in a paper to be published in the Astrophysical Journal and posted online today at arXiv.org. It is also linked to this release. Coauthors include associate research scientist Eugenio Rivera of UC Santa Cruz; associate astronomer Nader Haghighipour of the University of Hawaii-Manoa; and research scientists Gregory Henry and Michael Williamson of Tennessee State University.

The paper reports the discovery of two new planets around the nearby red dwarf star Gliese 581. This brings the total number of known planets around this star to six, the most yet discovered in a planetary system other than our own solar system. Like our solar system, the planets around Gliese 581 have nearly circular orbits.

The most interesting of the two new planets is Gliese 581g, with a mass three to four times that of the Earth and an orbital period of just under 37 days. Its mass indicates that it is probably a rocky planet with a definite surface, and that it has enough gravity to hold on to an atmosphere, according to Vogt.

Gliese 581, located 20 light years away from Earth in the constellation Libra, has a somewhat checkered history of habitable-planet claims. Two previously detected planets in the system lie at the edges of the habitable zone, one on the hot side (planet c) and one on the cold side (planet d). While some astronomers still think planet d may be habitable if it has a thick atmosphere with a strong greenhouse effect to warm it up, others are skeptical. The newly discovered planet g, however, lies right in the middle of the habitable zone.

"We had planets on both sides of the habitable zone--one too hot and one too cold--and now we have one in the middle that's just right," Vogt said.

The planet is tidally locked to the star, meaning that one side is always facing the star and basking in perpetual daylight, while the side facing away from the star is in perpetual darkness. One effect of this is to stabilize the planet's surface climates, according to Vogt. The most habitable zone on the planet's surface would be the line between shadow and light (known as the "terminator"), with surface temperatures decreasing toward the dark side and increasing toward the light side.

"Any emerging life forms would have a wide range of stable climates to choose from and to evolve around, depending on their longitude," Vogt said.

The researchers estimate that the average surface temperature of the planet is between -24 and 10 degrees Fahrenheit (-31 to -12 degrees Celsius). Actual temperatures would range from blazing hot on the side facing the star to freezing cold on the dark side.

If Gliese 581g has a rocky composition similar to the Earth's, its diameter would be about 1.2 to 1.4 times that of the Earth. The surface gravity would be about the same or slightly higher than Earth's, so that a person could easily walk upright on the planet, Vogt said.

The new findings are based on 11 years of observations of Gliese 581 using the HIRES spectrometer (designed by Vogt) on the Keck I Telescope at the W. M. Keck Observatory in Hawaii. The spectrometer allows precise measurements of a star's radial velocity (its motion along the line of sight from Earth), which can reveal the presence of planets. The gravitational tug of an orbiting planet causes periodic changes in the radial velocity of the host star. Multiple planets induce complex wobbles in the star's motion, and astronomers use sophisticated analyses to detect planets and determine their orbits and masses.

"It's really hard to detect a planet like this," Vogt said. "Every time we measure the radial velocity, that's an evening on the telescope, and it took more than 200 observations with a precision of about 1.6 meters per second to detect this planet."

To get that many radial velocity measurements (238 in total), Vogt's team combined their HIRES observations with published data from another group led by the Geneva Observatory (HARPS, the High Accuracy Radial velocity Planetary Search project).

In addition to the radial velocity observations, coauthors Henry and Williamson made precise night-to-night brightness measurements of the star with one of Tennessee State University's robotic telescopes. "Our brightness measurements verify that the radial velocity variations are caused by the new orbiting planet and not by any process within the star itself," Henry said.

The researchers also explored the implications of this discovery with respect to the number of stars that are likely to have at least one potentially habitable planet. Given the relatively small number of stars that have been carefully monitored by planet hunters, this discovery has come surprisingly soon.

"If these are rare, we shouldn't have found one so quickly and so nearby," Vogt said. "The number of systems with potentially habitable planets is probably on the order of 10 or 20 percent, and when you multiply that by the hundreds of billions of stars in the Milky Way, that's a large number. There could be tens of billions of these systems in our galaxy."

-NSF-