一颗巨大的蓬松行星绕着一颗凉爽的红矮星运行

国家天文台的基特峰望远镜有助于确定这一点[{” attribute=””>Jupiter-like Planet is the lowest-density gas giant ever detected around a red dwarf.

A gas giant exoplanet with the density of a marshmallow has been detected in orbit around a cool red dwarf star. A suite of astronomical instruments was used to make the observations, including the NASA-funded NEID radial-velocity instrument on the WIYN 3.5-meter Telescope at Kitt Peak National Observatory, a Program of NSF’s NOIRLab. Named TOI-3757 b, the exoplanet is the fluffiest gas giant planet ever discovered around this type of star.

Using the WIYN 3.5-meter Telescope at Kitt Peak National Observatory in Arizona, astronomers have observed an unusual Jupiter-like planet in orbit around a cool red dwarf star. Located in the constellation of Auriga the Charioteer around 580 light-years from Earth, this planet, identified as TOI-3757 b, is the lowest-density planet ever detected around a red dwarf star and is estimated to have an average density akin to that of a marshmallow.

Red dwarf stars are the smallest and dimmest members of so-called main-sequence stars — stars that convert hydrogen into helium in their cores at a steady rate. Although they are “cool” compared to stars like our Sun, red dwarf stars can be extremely active and erupt with powerful flares. This can strip orbiting planets of their atmospheres, making this star system a seemingly inhospitable location to form such a gossamer planet.

Shubham Kanodia，卡内基科学研究所地球与行星实验室研究员，论文第一作者 占星杂志至。 到目前为止，这只有在多普勒调查的小样本中才能看到，这些调查通常会发现远离这些红矮星的巨行星。 到目前为止，我们还没有足够大的行星样本来可靠地发现附近的气体行星。”

TOI-3757 b 周围仍然存在无法解释的谜团，其中主要是一颗气态巨行星如何围绕一颗红矮星，尤其是一颗低密度行星形成。 然而，Kanodia 团队相信他们可能有办法解决这个谜团。

在 NSF NOIRLab 的一个项目 Kit Peak 国家天文台 (KPNO) 的场地上，3.5 米的 Wisconsin-Indiana-Yale-NOIRLab (WIYN) 望远镜似乎正在观察从地平线溢出的银河。 带红色的大气眩光是一种自然现象，也为地平线着色。 KPNO 位于 Tohono O’odham 国家的亚利桑那索诺兰沙漠，这张银河系平面部分的清晰视图显示了在这种环境中观察微弱天体所需的有利条件。 这些条件包括低水平的光污染、20° 黑暗的天空和干燥的大气条件，使 WIYN 联盟的研究人员能够使用 WIYN 3.5 米跟踪对星系、星云和系外行星以及许多其他天文目标的观测望远镜及其姊妹 WIYN 0.9 米望远镜。 学分：KPNO/NOIRLab/NSF/AURA/R. Sparks

他们认为 TOI-3757 b 的极低密度可能是两个因素的结果。 第一个与行星的岩石核心有关； 人们认为，气态巨行星最初是巨大的岩石核心，其质量约为地球质量的十倍，此时它们会迅速吸收附近的大量气体，形成我们今天看到的气态巨行星。 TOI-3757b 的重元素丰度低于其他拥有气态巨行星的 M 矮星，这可能导致核心形成速度变慢，延缓了气体聚集的开始，从而影响了行星的整体密度。

第二个因素可能是行星的轨道，暂时认为它是略呈椭圆形的。 有时它比其他时候更接近它的恒星，导致显着的过度加热，从而导致行星的大气层膨胀。

美国宇航局用于系外行星调查的过境卫星 ([{” attribute=””>TESS) initially spotted the planet. Kanodia’s team then made follow-up observations using ground-based instruments, including NEID and NESSI (NN-EXPLORE Exoplanet Stellar Speckle Imager), both housed at the WIYN 3.5-meter Telescope; the Habitable-zone Planet Finder (HPF) on the Hobby-Eberly Telescope; and the Red Buttes Observatory (RBO) in Wyoming.

TESS surveyed the crossing of this planet TOI-3757 b in front of its star, which allowed astronomers to calculate the planet’s diameter to be about 150,000 kilometers (100,000 miles) or about just slightly larger than that of Jupiter. The planet finishes one complete orbit around its host star in just 3.5 days, 25 times less than the closest planet in our Solar System — Mercury — which takes about 88 days to do so.

The astronomers then used NEID and HPF to measure the star’s apparent motion along the line of sight, also known as its radial velocity. These measurements provided the planet’s mass, which was calculated to be about one-quarter that of Jupiter, or about 85 times the mass of the Earth. Knowing the size and the mass allowed Kanodia’s team to calculate TOI-3757 b’s average density as being 0.27 grams per cubic centimeter (about 17 grams per cubic feet), which would make it less than half the density of Saturn (the lowest-density planet in the Solar System), about one quarter the density of water (meaning it would float if placed in a giant bathtub filled with water), or in fact, similar in density to a marshmallow.

“Potential future observations of the atmosphere of this planet using NASA’s new James Webb Space Telescope could help shed light on its puffy nature,” says Jessica Libby-Roberts, a postdoctoral researcher at Pennsylvania State University and the second author on this paper.

“Finding more such systems with giant planets — which were once theorized to be extremely rare around red dwarfs — is part of our goal to understand how planets form,” says Kanodia.

The discovery highlights the importance of NEID in its ability to confirm some of the candidate exoplanets currently being discovered by NASA’s TESS mission, providing important targets for the new James Webb Space Telescope (JWST) to follow up on and begin characterizing their atmospheres. This will in turn inform astronomers what the planets are made of and how they formed and, for potentially habitable rocky worlds, whether they might be able to support life.

Reference: “TOI-3757 b: A low-density gas giant orbiting a solar-metallicity M dwarf” by Shubham Kanodia, Jessica Libby-Roberts, Caleb I. Cañas, Joe P. Ninan, Suvrath Mahadevan, Gudmundur Stefansson, Andrea S. J. Lin, Sinclaire Jones, Andrew Monson, Brock A. Parker, Henry A. Kobulnicky, Tera N. Swaby, Luke Powers, Corey Beard, Chad F. Bender, Cullen H. Blake, William D. Cochran, Jiayin Dong, Scott A. Diddams, Connor Fredrick, Arvind F. Gupta, Samuel Halverson, Fred Hearty, Sarah E. Logsdon, Andrew J. Metcalf, Michael W. McElwain, Caroline Morley, Jayadev Rajagopal, Lawrence W. Ramsey, Paul Robertson, Arpita Roy, Christian Schwab, Ryan C. Terrien, John Wisniewski and Jason T. Wright, 5 August 2022, The Astronomical Journal.
DOI: 10.3847/1538-3881/ac7c20