成果发布 | Imaging of the Nearest Cold Super-Jupiter by JWST

Due to the discovery of long-term acceleration in the radial velocity data of eps Ind A, Markus Janson from Stockholm University in Sweden suspected the presence of a giant planet around eps Ind A as early as 2009. “Thanks to the unprecedented astrometric precision of Gaia space telescope and the 24-year time gap between Hipparcos (a precursor to Gaia) and Gaia, I confirmed this giant planet as the nearest Jupiter-like planet through a combined analysis of an updated radial velocity data set and data from Hipparcos and Gaia,” said Fabo Feng, an associate professor at the Tsung-Dao Lee Institute of Shanghai Jiao Tong University.

Immediately after the discovery of Epsilon Indi A b, multiple international groups recognized that this giant planet is an ideal target for mid-infrared imaging due to its proximity and large angular distance from its host star (more than 1 arcsecond). This led to the first effort to image Epsilon Indi A b in the mid-infrared using the NaCo and NEAR instruments installed at the Very Large Telescope (VLT) from 2018 to 2019. Although no planets were detected by imaging, the team found that the planetary signal is probably quite close to the instruments' detection limits and placed constraints on the planet's age and mass.

Before the launch of JWST, Elisabeth Matthews from the Max Planck Institute for Astronomy (MPIA) and a group of astronomers, including Fabo Feng, secured more than 10 hours of observation time in JWST Cycle 1. Epsilon Indi A b was among the first targets observed by JWST after its launch in 2021. The JWST/MIRI image of Epsilon Indi A showed a strong signal in a position significantly different from that predicted by the solutions based on combined analyses of radial velocity and astrometric data. The group excluded the possibility of the signal being from background galaxies or other sources. Moreover, a reanalysis of the previous VLT/NEAR data revealed a weak signal with a significance of about 3 sigma, consistent with a planetary origin.

Hence, this signal is very likely to be a planet. The question now is whether we should call this planet Epsilon Indi A b or A c. Is this imaged planet the same as  Epsilon Indi A b?  “By analyzing combinations of different radial velocity and astrometric data sets, we find that the discrepancy between the previous prediction and the observed position of Epsilon Indi A b mainly comes from the shorter time span of previous radial velocity data sets. With updated radial velocity with longer time span, we are able to predict the correct planetary position that JWST revealed,” said Fabo Feng with confidence.

The successful imaging of Epsilon Indi A b opens a new window for characterizing the atmospheres of cold giant planets. It also demonstrates the importance of international collaborations and long-term efforts in making major breakthroughs and deepening our understanding of the universe.

由于在eps Ind A 的视向速度数据中发现了长期的加速运动，瑞典斯德哥尔摩大学的Markus Janson教授早在2009年就怀疑eps Ind A周围存在一颗巨行星。上海交通大学李政道研究所副教授冯发波表示：“得益于欧空局的Gaia空间望远镜前所未有的天体测量精度，以及Hipparcos（Gaia的前身）和Gaia之间24年的时间间隔，通过对更新的视向速度数据集以及Hipparcos和Gaia的数据进行综合分析，我在2019年确认了这颗巨行星是距离我们最近的类木行星。”

在发现eps Ind A b 后，国际研究团队意识到这颗巨行星由于其离地球很近且与主星的角距离大（超过1角秒），是中红外成像的理想目标。2018年至2019年间，一个国际团队使用甚大望远镜（VLT）上的NaCo和NEAR仪器首次尝试在地面对eps Ind A b进行中红外成像。尽管成像未能探测到显著的行星信号，但团队发现该行星可能已经接近仪器检测极限的边缘，并对行星的年龄和质量进行了约束。

在JWST发射之前，德国马普天文所（MPIA）的Elisabeth Matthews博士领导了一个包括冯发波在内的天文学家团队，成功申请到了JWST Cycle 1的超过10小时的观测时间。eps Ind A b 是JWST在2021年发射后观测的首批目标之一。JWST/MIRI的eps Ind A 的图像显示出一个很强的信号，不过其位置与基于视向速度和天体测量数据综合分析的预测位置显著不同。团队排除了该信号来自背景星系或其他来源的可能性。此外，对之前VLT/NEAR数据的重新分析后发现了一个约3 sigma显著性的微弱信号，与行星来源一致。

这张图总结了使用JWST/MIRI进行观测的过程，这些观测导致了eps Ind Ab 的成像。插图展示了在中红外波长10.65微米（左）和15.55微米（右）获得的MIRI图像的裁剪版本，显示了围绕eps Ind A 的区域，其位置由星形符号指示。星冕仪阻挡了来自恒星的光，在扣除这些来自恒星的光之后，一个新的源在左上方变得清晰可见。这个源就是系外行星eps Ind Ab。背景图像来自AllWISE天空巡天。© T. Müller (MPIA/HdA), E. Matthews (MPIA)

因此，这个信号很可能是一颗行星。现在的问题是我们应该将这颗行星称为eps Ind A b 还是 A c。这颗成像的行星是eps Ind A b 吗？“通过分析不同的视向速度和天体测量数据集的组合，我们发现之前的预测与eps Ind A b 成像位置之间的差异主要来源于先前分析中所用的视向速度数据集的时间跨度较短。通过分析更长时间跨度的视向速度数据，我们能够成功预测出JWST所揭示的正确行星位置，”冯发波满有信心地说道。

对eps Ind A b 的成功成像为描述冷气体巨行星的大气层开辟了新的窗口。这也展示了国际合作和长期努力在取得重大突破和加深我们对宇宙理解中的重要性。