NASA Says Data Reveals Kepler 452b, an Earth-Like Planet
By DENNIS OVERBYE

Inching ahead on their quest for what they call Earth 2.0, astronomers from NASA’s Kepler planet-hunting spacecraft announced on Thursday that they had found what might be one of the closest analogues to our own world yet.

It is a planet a little more than one and a half times as big in radius as Earth. Known as Kepler 452b, it circles a sunlike star in an orbit that takes 385 days, just slightly longer than our own year, putting it firmly in the “Goldilocks” habitable zone where the temperatures are lukewarm and suitable for liquid water on the surface — if it has a surface.

The new planet’s size puts it right on the edge between being rocky like Earth and being a fluffy gas ball like Neptune, according to studies of other such exoplanets. In an email, Jon Jenkins of NASA’s Ames Research Center, home of the Kepler project, and lead author of a paper being published in The Astronomical Journal, said the likelihood of the planet’s being rocky was 50 percent to 62 percent, depending on uncertainties in the size of its home star. That would mean its mass is about five times that of Earth.

Such a planet would probably have a thick, cloudy atmosphere and active volcanoes, Dr. Jenkins said, and twice the gravity of Earth. Describing the planet during a news conference, Dr. Jenkins lapsed into lines from John Keats’s poem “On First Looking Into Chapman’s Homer”: “Then felt I like some watcher of the skies / When a new planet swims into his ken.”

The star that lights this planet’s sky is about 1.5 billion years older than our sun and 20 percent more luminous, which has implications for the prospects of life, Dr. Jenkins said.

“We can think of Kepler-452b as an older, bigger cousin to Earth, providing an opportunity to understand and reflect upon Earth’s evolving environment,” he said. “It’s awe-inspiring to consider that this planet has spent six billion years in the habitable zone of its star, longer than Earth. That’s substantial opportunity for life to arise, should all the necessary ingredients and conditions for life exist on this planet.”

Asked if any radio telescopes had pointed at the planet to try to detect exterrestrial radio broadcasts, Dr. Jenkins said, “I hope so.”

To determine whether Kepler 452b deserves a place on the honor roll of possible home worlds, however, astronomers have to measure its mass directly, which requires being close enough to observe the wobbling of its star as it is tugged around by the planet’s gravity. For now that is impossible, as Kepler 452 is 1,400 light-years away.

The planet is the first to be confirmed in a new list of candidates unveiled by Kepler astronomers on Thursday. It brings the number of possible planets discovered by Kepler to 4,696, many of them small like Earth. “We are the bread crumbs of the universe,” said Jeff Coughlin, of the SETI Institute in Mountain View, Calif., who compiled the catalog.

The spacecraft, launched in 2009, spent four years staring at a patch of the Milky Way on the border between the constellations Cygnus and Lyra, looking for the dips in starlight caused by the passage of planets. Its pointing system failed in 2013, but astronomers are still analyzing the data Kepler collected. Every time they sift through it, new planets pop out.

In the meantime, Kepler has switched to a different mode of observing in a mission called K2.

The NASA news conference coincided with a major anniversary: It was only 20 years ago this fall that Michel Mayor and Didier Queloz, of the University of Geneva, discovered a planet circling the star 51 Pegasi, about 50 light-years from here. It was the first planet known to belong to a sunlike star outside our solar system, and its discovery ignited an astronomical revolution.

Dr. Queloz, who is now at the University of Cambridge in England, told the news conference, “This is a great time we live in.”

“If we keep working so well and so enthusiastically,” he went on, it is not too optimistic to think that in the future, “the issue of life on another planet will be solved.”

Astronomers say they now know from Kepler that about 10 percent of the 200 billion stars in the Milky Way have potentially habitable Earth-size planets, Kepler 452b probably among them. This means that of the 600 stars within 30 light-years of Earth, there are roughly 60 E.T.-class abodes, planets that could be inspected by a future generation of telescopes.

NASA 發現「另一顆地球」

美國太空總署（NASA）廿三日召開視訊記者會，宣布發現一顆猶如「第二個地球」的太陽系外行星，取名為克卜勒四五二b，距離地球一千四百光年。這顆行星和它繞行恆星的距離，與地球和太陽的距離相同，且位於較宜人居的地帶。

新發現的好消息是新行星不會太熱或太冷，可能有水。繞行的恆星和太陽差不多大，亮度多百分之十。不過恆星比太陽早十五億年出現，現已進入衰亡階段，溫度不斷升高，將蒸發行星表面所有的水分。科學家表示，觀測新行星下場，可以預知地球十億年後處境。

NASA指出：「系外行星，特別是如地球大小的其他世界，在廿一年前還是科幻小說範疇。經數千個新發現後，天文學家終於發現人類數千年來夢寐以求的：另一個地球。」

斥資六億美元（約台幣一百八十七億元）的「克卜勒太空望遠鏡計畫」自2009年五月啟動，目標在尋找銀河中的其他行星系，尤其是性質和地球相近，有液態水，環繞其他恆星運行的「適宜人居」行星。英國天文物理學家霍金廿日也宣布將斥資一億美元發展十年的探索外太空生物計畫。

克卜勒太空望遠鏡利用行星通過其環繞恆星正前方遮蔽其光芒的時機，觀測目標行星的亮度，藉以推測星體狀態。計畫啟始以來已觀測到4661顆行星，並確認其中1028顆。去年四月發現最像地球的「克卜勒186F」，距離地球有五百光年之遙，就算有生命適合居住，人類也到不了。

原文參照：
http://www.nytimes.com/2015/07/24/science/space/kepler-data-reveals-what-might-be-best-goldilocks-planet-yet.html

2015-07-24．聯合報．A1．要聞．編譯莊蕙嘉