The hunt for exoplanets in our galaxy is a deeply important endeavour. The more exoplanets we find, the better we can understand our own Solar System - and how life emerges in the Universe. To date, over 4,000 exoplanets have been confirmed - but a new discovery could widen the search, helping us to find exoplanets that previously have proven too difficult to detect.
於咱們銀河系中，找尋系外行星是一項非常重要的嘗試。發現愈多的系外行星，人們愈能深入瞭解咱們太陽系，及生命如何出現於宇宙中。迄今為止，已經證實超過4千顆系外行星，不過一項新發現可能擴大此搜尋。這有助於人們找到，先前已經證實很難發現的系外行星。
The newly discovered exoplanet, with a mass comparable to that of Saturn, orbits a very small, cool red dwarf right at the lower mass limit for main sequence stars, and located about 35 light-years away. However, it's not just the planet, nor the star, that's so groundbreaking here.
這顆新近被發現、具有比得上土星質量的系外行星圍繞一顆，座落於大約35光年外、正好處於主序恆星(任何正在其核心中，融合氫且具有來自核心核融合之向外壓力及向內推壓之重力穩定的平衡的恆星)質量下限的很小冷紅矮星，作繞軌運行。不過，在此這完全不是該行星，也不是該恆星，這麼具突破性。
What's particularly special in this discovery is how astronomers used a radio telescope to track the movement of the star through the Milky Way, and identify the snaking wiggle in that movement as the star is gravitationally affected by an orbiting exoplanet. This very tricky accomplishment is called the astrometric technique, and it's the first time it's been successfully deployed using a radio telescope.
在此發現中的特別之處是，天文學家們如何利用射電望遠鏡，來追跡該恒星在銀河系中的運行，及確認在那運行中的蛇形擺動，是該恆星受到一顆，繞軌運行之系外行星的重力影響。這種非常需要技巧的技能，被稱為天體測量學技術，且這是首度使用射電望遠鏡，成功利用此技能。
Using an orbital wobble to detect an exoplanet is not a new idea. You see, the orbital centre of a planetary system isn't in the middle of the star. Rather, all bodies in the system orbit a mutual centre of gravity, called the barycentre. The barycentre of the Solar System, for example, is just outside the surface of the Sun, mainly due to the gravitational influence of Jupiter and Saturn.
利用軌道的擺動來發現系外行星，並非新構想。行星系的軌道中心，不在恒星中央。而是系中的所有天體皆圍著一個，被稱為引力中心的共同重力中心，作繞軌運行。譬如，太陽系的引力中心，就在太陽表面之外，主要是由於木星及土星的重力影響。
When we're looking at other stars with massive, closely orbiting exoplanets, this effect can be detected in the way light wavelengths are stretched or compressed as the star moves around. This detection technique is called Doppler spectroscopy, or the radial velocity method, and it's one of the more common methods for finding exoplanets.
當人們探究具有大質量、接近地作繞軌運行之系外行星的其他恆星時，這種效應在恒星運行時，光波長被拉長或壓縮的方式中，能被探測出。這種探測技術被稱為都卜勒光譜技術(或徑向速度測量法)，且這是找尋系外行星較常見的方法之一。
The astrometric technique is a little different. The Milky Way's stars aren't fixed in space; they move around the galaxy, and the study of this movement is called astrometry. So, rather than using changes in wavelengths, the astrometric technique looks for deviations from a straight line of movement.
上述天體測量學技術略有不同。銀河系的恆星並非固定於太空中，它們在銀河系中運行，而研究此運行，被稱為天體測量學。因此，這種天體測量學技術，不是使用波長的變化，而是從運行的直線找尋偏差。
This method can be used to detect exoplanets that Doppler spectroscopy can't, such as exoplanets circling in larger orbits around their stars.
此方法能被用來發現，都卜勒光譜技術無法發現的系外行星，諸如以更大軌道環繞其恆星運行的系外行星。
"Our method complements the radial velocity method, which is more sensitive to planets orbiting in close orbits, while ours is more sensitive to massive planets in orbits further away from the star," said astrophysicist Gisela Ortiz-Leon of the Max Planck Institute for Radio Astronomy in Germany.
德國馬克斯普朗克射電天文學研究所，天體物理學家Gisela Ortiz-Leon宣稱：「他們的方法補充了，對於在近軌道上，作繞軌運行之行星更敏感的徑向速度測量法。同時，他們的方法對更遠離恆星之軌道上的大質量行星更為敏感。
Indeed, these other techniques have found only a few planets with characteristics such as planet mass, orbital size, and host star mass, similar to the planet we found. We believe that the VLBA, and the astrometry technique in general, could reveal many more similar planets."
實際上，上述其他技術僅已經發現少數，具有諸如行星質量、軌道大小及宿主恆星質量等特徵，類似他們發現的行星。他們認為，(射電望遠鏡)超長基線陣列(VLBA：Very Long Baseline Array，也就是一般的天體測量學技術)能揭露更多類似的行星。」
The VLBA is the Very Long Baseline Array, a network of 10 radio antennas widely distributed across the US. For 18 months starting in June 2018, the research team, led by astronomer Salvador Curiel of the National Autonomous University of Mexico, tracked a small star called TVLM 513-46546 across space for a year and a half.
VLBA是一個，由十座廣佈於美國之射電天線組成的超長基線陣列網絡。由墨西哥國立自治大學天文學家，Salvador Curiel領導的研究團隊，從2018年6月開始追蹤，太空中一顆被稱為TVLM 513-46546的小恆星，達18個月。
Painstaking and careful analysis of the data revealed that the star was not travelling in a perfectly straight line, but was travelling more of a snaking path. The periodicity and amplitude of the wiggle revealed a planet on a 221-day orbit, and between 38 and 46 percent of the mass of Jupiter - a little more massive than Saturn, which is around 30 percent of the mass of Jupiter.
經過悉心、謹慎的分析此些數據揭露，該恆星並非以完美直線運行，而是以較為蛇行的途徑運行。此擺動的周期性及振幅揭露了一顆，位於221天的軌道上，質量介於木星38％至46％的行星。此質量稍大於土星，土星質量大約是木星的30％。
圖1. 蛇形擺動的軌道 (圖援用自原文)
"Giant planets, like Jupiter and Saturn, are expected to be rare around small stars like this one, and the astrometric technique is best at finding Jupiter-like planets in wide orbits, so we were surprised to find a lower mass, Saturn-like planet in a relatively compact orbit. We expected to find a more massive planet, similar to Jupiter, in a wider orbit," Curiel said.
Curiel宣稱：「類似木星及土星的巨大行星圍繞像上述小恆星，被預期是罕見的。而天體測量學技術最適合找尋寬軌道中，類似木星的行星。因此，他們感到訝異，在相對緊湊之軌道上，發現了一顆類似土星的較低質量行星。他們期盼在更寬的軌道上，發現一顆類似木星的更大質量行星。」
The astrometric technique is more commonly used to study binary stars, whose gravitational effect on each other is much more pronounced than the effect of a planet on a star. Only once before has the astrometric technique been used to discover an exoplanet (although it has been used to study already-known exoplanets), and never before with a radio telescope.
該種天體測量學技術較常被用來研究，彼此重力影響比一顆行星對一顆恆星之影響，更明顯得多的雙星。先前只有一次曾使用該種技術，來發現一顆系外行星(雖然它已經被用來研究已知的系外行星)，且先前不曾使用射電望遠鏡。
Earlier this year, however, another group of scientists announced the first use of a radio telescope to detect an exoplanet. It wasn't through astrometry, but by detecting the circular polarisation of radio waves generated by a planet's movement through a red dwarf's magnetic field.
不過，今年(2020年)早些時候，另一支科學家團隊宣佈，首次使用射電望遠鏡，來探測一顆系外行星。這不是透過天體測量學，而是藉由探測，因行星運行通過一顆紅矮星的磁場，所產生之射電波的圓極化(這是一種極化狀態，在此狀態中，這種波之電磁場中的每個點，具有恆定的幅度，但其方向在垂直於這種波之方向的平面中，以恆定速率旋轉)。
So, although the detection was quite challenging for Curiel's team, their ultimate success confirms the promise of both radio telescopes and the astrometric technique in finding planets that other techniques miss.
因此，對Curiel的團隊而言，雖然這種探測相當具有挑戰性。不過，他們最終的成功證實了，射電望遠鏡與天體量測學技術，在找尋其他技術遺漏之行星上的指望。
The Gaia telescope is currently surveying the Milky Way, creating the most detailed and accurate astrometric map of the galaxy yet; it's expected that this data will blow astrometric exoplanet detection wide open, with an estimated tens of thousands of exoplanet discoveries to come.
目前，正進行勘測銀河系的蓋亞太空望遠鏡產生了，迄今為止有關銀河系，最詳細且最準確的天體測量學圖。預期此數據將充分開啟，天體量測學的系外行星探測之門，估計會發現數萬系外行星。
原文網址：https://www.sciencealert.com/for-the-first-time-a-radio-telescope-has-been-used-to-find-a-planet-by-its-wiggling-star
翻譯：許東榮
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