독일 하이델베르크(Heidelberg)에 있는 막스 플랑크 천문학 연구소(Max Planck Institute for Astronomy, MPIA)의 로라 크라이드버그(Laura Kreidberg)와 토마스 미칼-에반스(Thomas Mikal-Evans)를 포함한 천문학자들은 NASA의 허블 우주 망원경(HST)을 사용해 외계행성 ‘GJ 9827d’의 대기에서 수증기를 감지했다고 밝혔다. 지구의 지름의 약 두 배 크기인 GJ 9827는 우리 은하계 외에도 물이 풍부한 행성이 있을 수 있다는 가능성을 보여준다. 

그러나 행성의 대기가 대부분 물로 이루어져 있는 것인지, 아니면 수소가 풍부한 대기에서 소량의 수증기를 분광법으로 측정한 것인지, 원시 수소·헬륨 대기가 항성 복사에 의해 증발한 후 남겨진 것인지는 아직 분명하지 않다.

그러나 GJ 9872d의 나이가 약 60억 년이라는 점과 모항성과의 근접성을 감안할 때, GJ 9872d는 강렬한 조사로 인해 원시 수소의 대부분을 잃었을 것이며, 수증기가 풍부한 대기가 남아 있을 가능성이 높다. 또한 수소 신호를 감지하려는 시도는 지금까지 모두 실패했다.

이 행성은 화씨 800도(섭시 400)에 달하는 금성만큼 뜨겁기 때문에, 대기가 주로 수증기로 이뤄져 있다면 척박하고, 증기로 뒤덮인 행성이 될 것이다. 

베네케는 “현재 두 가지 시나리오가 있다. 첫 번째는 GJ 9827d가 물이 포함된 수소 대기에 둘러쌓여 작은 해왕성을 만드는 것이다. 또는 목성의 위성 유로파의 따뜻한 버전일 수 있는데, 유로파는 지각 아래에 지구보다 두 배 많는 물을 가지고 있다“고 말했다. 

베네케는 이어 “GJ 9827d 행성은 반은 물이고 반은 암석일 수 있다. 그리고 더 작은 부분을 차지하는 암석체 위에는 많은 수증기가 있을 것이다”라고 덧붙였다. 

만약 행성에 물이 풍부한 대기가 남아 있다고 가정했을 때, 그 행성은 모항성으로부터 현재 위치보다 더 멀리 떨어진 곳에서 형성됐을 것이다. 그곳의 온도는 현재 위치보다 낮고, 물은 얼음으로 존재한다. 이 시나리오에서 GJ 9827d는 항성에 더 가까이 이동했을 것이고, 더 많은 조사를 받아 얼음을 물과 증기로 바꾸었을 것이다. 수소는 가열되어 빠져나갔거나, 행성의 약한 중력을 벗어나는 과정에 있을 것이다. 

또 다른 이론은 행성이 뜨거운 항성 가까이에서 형성됐으며 대기에 미량의 물의 흔적이 있다는 것이다.

허블 프로그램은 3년 동안 11번의 통과(행성이 항성 앞을 가로지르는 사건)를 통해 행성을 관측했다. 통과하는 동안 별빛은 행성의 대기를 통해 걸러지고 물 분자의 스펙트럼 지문을 갖는다. 

MPIA의 천문학자들을 포함한 천문학자들은 최근 제임스 웹 우주 망원경으로 GJ 9827d를 관측했다. 토마스는 ”제임스 웹 우주 망원경(James Webb Space Telescope)은 일산화탄소, 이산화탄소, 메탄과 같은 탄소 함유 분자를 포함한 추가적인 적외선 관찰로 훨씬 더 많은 것을 볼 수 있다“며 “행성을 구성하는 원소를 파악하면 이 원소를 그들이 공전하는 별의 원소와 비교해 행성이 어떻게 형성되었는지 이해할 수 있다“고 말했다. 

그들은 물의 신호와 다른 분자를 찾았다. 크라이드버그는 “우리는 이러한 데이터가 무엇을 밝혀낼지 기대된다”며 “천문학자들은 물 세계의 문제를 완전히 해결할 수 있기를 바란다”고 말했다.

한편, GJ 9827d는 2017년 NASA의 케플러 우주 망원경에 의해 발견됐다. 이 행성은 6.2일마다 적색왜성 주위를 한 바퀴 도는 궤도를 가지고 있다. 

[원문보기] 

Hubble finds water vapour in small exoplanet's atmosphere
Steamy world could be a sample of water-rich planets throughout our galaxy

 Astronomers are intrigued when finding evidence of water vapour on exoplanets. Based on observations with the Hubble Space Telescope, a recent example is the planet GJ 9827d, which may have a water-rich atmosphere around it. Water is one of the most common molecules in the Universe, and all life on Earth requires it. It functions as a superior solvent and enables critical chemical reactions in biological cells. No bigger than twice Earth’s diameter, GJ 9827d could be an example of potential water-rich worlds elsewhere in our galaxy. But don’t plan on buying real estate there. The planet is as hot as Venus, which makes it a steamy world.

Using the Hubble Space Telescope (HST), astronomers, including Laura Kreidberg and Thomas Mikal-Evans from the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, observed the smallest exoplanet where scientists detected water vapour in the atmosphere. This planet, GJ 9872d, is located 97 light-years away in the constellation Pisces.

Water detected on the smallest exoplanet so far

“This would be the first time that we can directly show through atmospheric detection, that these planets with water-rich atmospheres can actually exist around other stars,” said team member Björn Benneke of the Trottier Institute for Research on Exoplanets at Université de Montréal, Canada. “This is an important step toward determining the prevalence and diversity of atmospheres on rocky planets.”

“Water on a planet this small is a landmark discovery,” added Laura Kreidberg, MPIA’s director of the APEx (Atmospheric Physics of Exoplanets) department. She is the co-principal investigator of the underlying HST observing program. “It pushes closer than ever to characterizing truly Earth-like worlds.”

However, it remains too early to tell whether Hubble spectroscopically measured a small amount of water vapour in a puffy hydrogen-rich atmosphere or if the planet’s atmosphere is mostly made of water, left behind after a primaeval hydrogen/helium atmosphere evaporated under stellar radiation.

Water world or hydrogen-rich mini-Neptune?

“Our observing program, led by principal investigator Ian Crossfield of Kansas University in Lawrence, Kansas, was designed specifically with the goal to not only detect the molecules in the planet’s atmosphere but to actually look specifically for water vapour. Either result would be exciting, whether water vapour is dominant or just a tiny species in a hydrogen-dominant atmosphere,” said the science paper’s lead author, Pierre-Alexis Roy of the Trottier Institute for Research on Exoplanets at Université de Montréal, Canada.

However, given its age of approximately six billion years and proximity to its host star, GJ 9872d should have lost most of its primordial hydrogen due to the intense irradiation, likely leaving behind an atmosphere dominated by water vapour. In addition, any attempt to detect hydrogen signatures failed so far.

“Until now, we had not been able to detect the atmosphere of such a small planet directly. And we’re slowly getting in this regime now,” added Benneke. “At some point, as we study smaller planets, there must be a transition where there’s no more hydrogen on these small worlds, and they have atmospheres more like Venus, which is dominated by carbon dioxide.”

Because the planet is as hot as Venus, at 400 degrees Celsius, it definitely would be an inhospitable, steamy world if the atmosphere were predominantly water vapour.

At present, the team is left with two possibilities. The planet is still clinging to a hydrogen-rich envelope laced with water, making it a mini-Neptune. Alternatively, it could be a warmer version of Jupiter’s moon Europa, which has twice as much water as Earth beneath its crust. “The planet GJ 9827d could be half water, half rock. And there would be a lot of water vapour on top of some smaller rocky body,” said Benneke.

Suppose the planet has a residual water-rich atmosphere. In that case, it must have formed farther away from its host star, where the temperature is lower than at its present location and water is available as ice. In this scenario, the GJ 9827d would have migrated closer to the star and received more radiation, turning the ice into water and vapour. The hydrogen was heated and escaped or is still in the process of escaping the planet’s weak gravity. The alternative theory is that the planet formed close to the hot star, with a trace of water in its atmosphere.

A perfect target for Hubble and Webb

The Hubble program observed the planet during 11 transits – events in which it crossed in front of its star – spaced out over three years. During transits, starlight is filtered through the planet’s atmosphere and has the spectral fingerprint of water molecules. If there are clouds on the planet, they are low enough in the atmosphere to not hide Hubble’s view of the atmosphere completely, and the telescope can probe water vapour above the clouds.

Hubble’s discovery opens the door to studying the planet in more detail. Therefore, astronomers, including those from MPIA, have recently observed GJ 9827d with the James Webb Space Telescope. They looked for water signatures and other molecules in more detail. “We can hardly wait to see what those data reveal,” Kreidberg says. “Hopefully, we can now settle the question of water worlds once and for all.”

[출처] = Max Planck Institute for Astronomy(https://www.mpia.de/news/science/2024-03-gj9827d-water-world#) / 2024년 1월 25일

[번역 = 박원희 기자]