新発見された惑星の一つ「TRAPPIST-1f」から見た光景。NASAが発表した想像図
地球から39光年離れた恒星の周りに、地球に似た7つの惑星を発見したと、ベルギーのリエージュ大学やアメリカ航空宇宙局（NASA）の国際チームが2月22日付の科学誌「ネイチャー」に発表した。
研究チームはNASAのスピッツァー宇宙望遠鏡を使って、「TRAPPIST（トラピスト）-1」という恒星を観測。恒星を周回する惑星が少なくとも7つ存在することを確認した。
7つの惑星の大きさは、地球の76％〜113％と大差ないのが特徴だ。岩石でできていると見られている。恒星から近い順に「TRAPPIST-1b」〜「TRAPPIST-1h」とアルファベット順に名前がついている。e、f、gの3つの惑星は、水が液体として存在しうる「ハビタブルゾーン」に位置しているという。
ワシントンポストによると、研究チームのリーダーを務めたリエージュ大学の天文物理学者マイケル・ギロン氏は「この恒星は非常に小さく低温のため、（周回する）惑星は温暖だ。つまり、惑星の表面には液体の水分や、生命までもが存在する可能性がある」と述べている。NASAの責任者も「生命を育むことができる第2の地球が見つかるかどうかは、もはや問題ではない。見つかるのは時間の問題だ」と述べ、今回の発見の意義を強調した。
今回の発見は、NASAが「太陽系外惑星に関する新しい重要な発見」について、2月22日に会見すると予告したことで、大きな注目を集めていた。
■惑星「TRAPPIST-1d」から見た光景の想像図（360度イメージ）
■関連スライドショー（ハッブル宇宙望遠鏡が捉えた宇宙）
ハッブル宇宙望遠鏡が捉えた宇宙
Hubble Captures View of 'Mystic Mountain'(01 of40)
Hubble\'s 20th anniversary image shows a mountain of dust and gas rising in the Carina Nebula. The top of a three-light-year tall pillar of cool hydrogen is being worn away by the radiation of nearby stars, while stars within the pillar unleash jets of gas that stream from the peaks.\r\nCredit: NASA, ESA, and M. Livio and the Hubble 20th Anniversary Team (STScI)
ACS Image of NGC 5866(02 of40)
Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)\r\nAcknowledgment: W. Keel (University of Alabama, Tuscaloosa)
Giant "Twisters" in the Lagoon Nebula(03 of40)
Credit: A. Caulet (ST-ECF, ESA) and NASA
The Spirograph Nebula (IC 418)(04 of40)
Credit: NASA and The Hubble Heritage Team (STScI/AURA)\r\nAcknowledgment: Dr. Raghvendra Sahai (JPL) and Dr. Arsen R. Hajian (USNO)\r\n
30 Doradus in Ultraviolet, Visible, and Red Light(05 of40)
Credit: NASA, ESA, F. Paresce (INAF-IASF, Bologna, Italy), R. O\'Connell (University of Virginia, Charlottesville), and the Wide Field Camera 3 Science Oversight Committee
The Ant Nebula (Menzel 3): Fiery Lobes Protrude From Dying, Sun-like Star(06 of40)
Image Credit: NASA, ESA and The Hubble Heritage Team (STScI/AURA)\r\nAcknowledgment: R. Sahai (Jet Propulsion Lab) and B. Balick (University of Washington)\r\n
The Eskimo Nebula (NGC 2392)(07 of40)
Credit: NASA, Andrew Fruchter and the ERO Team [Sylvia Baggett (STScI), Richard Hook (ST-ECF), Zoltan Levay (STScI)]
Dying Star HD 44179, the "Red Rectangle," Sculpts Rungs of Gas and Dust(08 of40)
Credit: NASA; ESA; Hans Van Winckel (Catholic University of Leuven, Belgium); and Martin Cohen (University of California, Berkeley)
Galaxy Triplet Arp 274(09 of40)
Arp 274 is a trio of galaxies. They appear to be partially overlapping in this image, but may be located at different distances.\r\nCredit: NASA, ESA, M. Livio and the Hubble Heritage Team (STScI/AURA)
Cassiopeia A: Colorful, Shredded Remains of Old Supernova(10 of40)
This youngest-known supernova remnant in our galaxy lies 10,000 light years away in the constellation Cassiopeia. The light from this exploding star first reached Earth in the 1600s.\r\nImage Credit: NASA and The Hubble Heritage Team (STScI/AURA)\r\nAcknowledgment: R. Fesen (Dartmouth) and J. Morse (Univ. of Colorado)
Rainbow Image of the Egg Nebula(11 of40)
An onionskin-like structure of concentric dust shells surround a central, aging star. Twin beams of light radiate from the star and illuminate the usually invisible dust. Artificial colors show how light reflects off the particles and heads toward Earth.\r\nCredit: NASA and The Hubble Heritage Team (STScI/AURA)\r\nAcknowledgment: W. Sparks (STScI) and R. Sahai (JPL)
Jet in Carina: WFC3 UVIS Full Field(12 of40)
Credit: NASA, ESA, and the Hubble SM4 ERO Team
Jet in Carina(13 of40)
Credit: NASA, ESA, and the Hubble SM4 ERO Team
Starburst Galaxy M82(14 of40)
Plumes of glowing hydrogen blast from the central nucleus of M82. The pale, star-like objects are clusters of tens to hundreds of thousands of stars.\r\nCredit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)\r\nAcknowledgment: J. Gallagher (University of Wisconsin), M. Mountain (STScI), and P. Puxley (National Science Foundation)
Picture Album: Hubble's Black and White View of the Universe(15 of40)
Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)\r\nAcknowledgment: A. Nota (STScI/ESA)
The Eagle Has Risen: Stellar Spire in the Eagle Nebula(16 of40)
A billowing tower of gas and dust rises from the stellar nursery known as the Eagle Nebula. This small piece of the Eagle Nebula is 57 trillion miles long (91.7 trillion km).\r\nCredit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)
Ring of Hot Blue Stars Pinwheels Around Yellow Nucleus of Hoag's Object Galaxy(17 of40)
Image Credit: NASA and The Hubble Heritage Team (STScI/AURA)\r\nAcknowledgment: Ray A. Lucas (STScI/AURA)\r\n
Nucleus of Galaxy Centaurus A(18 of40)
Credit: E.J. Schreier (STScI), and NASA
Saturn's Rings in Ultraviolet Light(19 of40)
Credit: NASA and E. Karkoschka (University of Arizona)
HST ACS/WFC Image of NGC 3021(20 of40)
Credit: NASA, ESA, and A. Riess (STScI/JHU)
NASA's Great Observatories Examine the Galactic Center Region(21 of40)
Credit: NASA, ESA, SSC, CXC, and STScI
Interacting Spiral Galaxies NGC 2207 and IC 2163(22 of40)
Image Credit: NASA, ESA, and The Hubble Heritage Team (STScI)\r\n
Combined X-Ray and Optical Images of the Crab Nebula(23 of40)
Credits for X-ray Image: NASA/CXC/ASU/J. Hester et al.\r\nCredits for Optical Image: NASA/HST/ASU/J. Hester et al.
Hubble's Sharpest View of the Orion Nebula(24 of40)
Thousands of stars are forming in the cloud of gas and dust known as the Orion nebula. More than 3,000 stars of various sizes appear in this image. Some of them have never been seen in visible light.\r\nCredit: NASA,ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team
Star-Forming Region S106(25 of40)
Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
A String of 'Cosmic Pearls' Surrounds an Exploding Star(26 of40)
Credit: NASA, ESA, P. Challis and R. Kirshner (Harvard-Smithsonian Center for Astrophysics)
A Perfect Storm of Turbulent Gases in the Omega/Swan Nebula (M17)(27 of40)
Credit: NASA, ESA and J. Hester (ASU)
Three Moons Cast Shadows on Jupiter(28 of40)
Credit: NASA, ESA, and E. Karkoschka (University of Arizona)
The Helix Nebula: a Gaseous Envelope Expelled By a Dying Star(29 of40)
Credit: NASA, ESA, C.R. O\'Dell (Vanderbilt University), M. Meixner and P. McCullough (STScI)
"Light Echo" Illuminates Dust Around Supergiant Star V838 Monocerotis (V838 Mon)(30 of40)
Credit: NASA and The Hubble Heritage Team (AURA/STScI)
The Cat's Eye Nebula: Dying Star Creates Fantasy-like Sculpture of Gas and Dust(31 of40)
The Cat\'s Eye Nebula, one of the first planetary nebulae discovered, also has one of the most complex forms known to this kind of nebula. Eleven rings, or shells, of gas make up the Cat\'s Eye.\r\nCredit: NASA, ESA, HEIC, and The Hubble Heritage Team (STScI/AURA)\r\nAcknowledgment: R. Corradi (Isaac Newton Group of Telescopes, Spain) and Z. Tsvetanov (NASA)
Optical and X-ray Composite Image of SNR 0509-67.5(32 of40)
Science Credit: NASA, ESA, and B. Schaefer and A. Pagnotta (Louisiana State University, Baton Rouge)\r\nImage Credit: NASA, ESA, CXC, SAO, the Hubble Heritage Team (STScI/AURA), and J. Hughes (Rutgers University)\r\n
Spiral Galaxy M74(33 of40)
Bright knots of glowing gas light up the arms of spiral galaxy M74, indicating a rich environment of star formation. Messier 74, also called NGC 628, is slightly smaller than our Milky Way.\r\nCredit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration\r\nAcknowledgment: R. Chandar (University of Toledo) and J. Miller (University of Michigan)
"X" Structure at Core of Whirlpool Galaxy (M51)(34 of40)
Credit: H. Ford (JHU/STScI), the Faint Object Spectrograph IDT, and NASA
Starburst Cluster Shows Celestial Fireworks(35 of40)
Credit: NASA, ESA, R. O\'Connell (University of Virginia), F. Paresce (National Institute for Astrophysics, Bologna, Italy), E. Young (Universities Space Research Association/Ames Research Center), the WFC3 Science Oversight Committee, and the Hubble Heritage Team (STScI/AURA)
A Giant Hubble Mosaic of the Crab Nebula(36 of40)
The Crab Nebula is a supernova remnant, all that remains of a tremendous stellar explosion. Observers in China and Japan recorded the supernova nearly 1,000 years ago, in 1054.\r\nCredit: NASA, ESA, J. Hester and A. Loll (Arizona State University)
The Majestic Sombrero Galaxy (M104)(37 of40)
A brilliant white core is encircled by thick dust lanes in this spiral galaxy, seen edge-on. The galaxy is 50,000 light-years across and 28 million light years from Earth.\r\nCredit: NASA and The Hubble Heritage Team (STScI/AURA)
Gas Pillars in the Eagle Nebula (M16): Pillars of Creation in a Star-Forming Region(38 of40)
Credit: NASA, ESA, STScI, J. Hester and P. Scowen (Arizona State University)
A Galactic Spectacle(39 of40)
Credit: NASA, ESA, SAO, CXC, JPL-Caltech, and STScI\r\nAcknowledgment: J. DePasquale (Harvard-Smithsonian CfA), and B. Whitmore (STScI)
Saturn's Double Light Show(40 of40)
Credit: NASA, ESA, and Jonathan Nichols (University of Leicester)
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