NASA’s planet hunting Transiting Exoplanet Survey Satellite (TESS) has discovered an Earth-sized world within the habitable range of its star, which could allow the presence of liquid water. Another first for TESS is its discovery of a world orbiting two stars, evoking the planet Tatooine from Star Wars, but unlikely to be hospitable to life as we know it.
The potentially habitable planet, named “TOI 700 d,” is relatively close to Earth — only 100 light years away, NASA’s Jet Propulsion Laboratory announced during the winter American Astronomical Society meeting in Honolulu, Hawaii on Jan. 6. Proxima Centauri, the closest star to Earth, is just over four light years away.
“TESS was designed and launched specifically to find Earth-sized planets orbiting nearby stars,” said Paul Hertz, NASA astrophysics division director. TESS initially misclassified the star, but several amateur astronomers identified the error. “When we corrected the star’s parameters, the sizes of its planets dropped, and we realized the outermost one was about the size of Earth and in the habitable zone,” said Emily Gilbert, a graduate student at the University of Chicago. The discovery was later confirmed by the Spitzer Space Telescope.
TESS stabilizes on one area of the sky to detect whether objects — planets — pass in front of stars, which causes a temporary drop in the stars’ luminosity. This allows TESS to infer the presence of a planet, its size and orbit. Star TOI 700 is small, about 40 percent of our Sun’s size and only about half as hot.
TESS discovered three planets in orbit, named TOI 700 b, c and d. Only “d” is in the so-called “Goldilocks zone,” not too far from and not too close to the star, where the temperature could allow the presence of liquid water. The name is an allusion to the fairy tale “Goldilocks and the Three Bears,” in which a young girl samples three bowls of porridge and finds that one is “just right — not too hot and not too cold.”
TOI 700 d is about 20 percent larger than Earth and orbits its star in 37 days. It receives 86 percent of the energy that Earth receives from the Sun. It remains to be seen what d is made of. Researchers have generated models based on the size and type of star in order to predict d’s atmospheric composition and surface temperature.
In one simulation, NASA explained, the planet is covered in oceans with a “dense, carbon-dioxide-dominated atmosphere similar to what scientists suspect surrounded Mars when it was young.” The planet is tidally locked to TOI 700, meaning that one side always faces the star, in the same way that Earthlings never see the “dark side” of the Moon. This synchronous rotation meant that, in another model, one side of the planet was constantly covered in clouds. A third simulation predicted an all-land world, where winds flow from the planet’s dark side to its light one.
Another discovery announced at the meeting was TESS’s first finding of an exoplanet orbiting two stars instead of one, also known as a circumbinary planet. The announcement prompted comparisons with Luke Skywalker’s home world of Tatooine in the “Star Wars” movie series, with its bewitching double sunsets. But the newly found planet’s size alone — it is 6.9 times larger than Earth, almost the size of Saturn — makes it unlikely to be livable.
Named TOI 1338 b, it is the only planet in the TOI 1338 system, which lies 1,300 light-years away in the constellation Pictor, and orbits its stars every 95 days. The two stars orbit each other every 15 days. One is about 10 percent bigger than our Sun, while the other is cooler, dimmer and only one-third the Sun’s mass.
在一月六日於夏威夷檀香山舉行的冬季美國天文學會會議中，美國航太總署的噴射推進實驗室宣布，這顆可能適合居住的行星被命名為「TOI 700 d」，距離地球相對接近──大約一百光年遠。不過，距離地球最近的恆星「半人馬座毗鄰星」只比四光年遠一點。
TESS發現三枚行星繞行這顆恆星，並將它們命名為「TOI 700 b」、「TOI 700 c」，以及「TOI 700 d」。其中，只有「TOI 700 d」位於所謂的「古迪洛克恆星適居區」中，距離恆星不會太遠也不會太近，溫度也能允許液態水存在。「古迪洛克」這個名稱的典故來自童話故事《古迪洛克與三隻小熊》。故事中，年輕的女孩試吃了三碗不同的麥片糊，發現有一碗「溫度剛剛好──不會太燙，也不會太涼。」
「TOI 700 d」體積約比地球大百分之二十，繞行恆星一圈的時間為三十七天，接收到的能量是地球從太陽接收的百分之八十六。該行星的結構仍有待進一步觀察。根據其宿主恆星的體積大小和類型，研究人員發展出幾個模式，以預測這顆行星的大氣組成和表面溫度。
在其中一個模擬模式中，NASA解釋，「TOI 700 d」被海洋所覆蓋，伴隨著「濃密、以二氧化碳為主的大氣層」，就像是科學家推測火星年輕時包覆其外層的大氣。該行星受恆星「TOI 700」潮汐鎖定，也就是其中一面永遠朝向太陽，就像地球人永遠看不到月球的暗面一般。在另一個模擬模式中，這樣的「同步自轉」導致該行星的另一面經常被雲霧遮蔽。第三個模擬模式則預測該行星全部是陸地，風從行星的黑暗面吹向明亮面。
該行星被命名為「TOI 1338 b」，是「TOI 1338」太陽系中唯一的行星，距離地球一千三百光年遠，位於星座「繪架座」中，每九十五天繞著雙恆星運轉一圈。兩顆恆星每十五天繞著彼此運行一周，其中一顆比我們的太陽大十分之一，另一顆溫度較低、亮度較暗，質量只有太陽的三分之一。
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