最も一般的な星型を持つ惑星は、人が住めない可能性がある。 Planets orbiting most common star type may be uninhabitable
2022-10-21 カリフォルニア大学リバーサイド校(UCR)
GJ1252bと名付けられたこの無大気惑星の発見につながった研究は、アストロフィジカル・ジャーナル・レターズ誌に詳しく紹介されている。
この惑星は、地球の1日の間に恒星の周りを2回公転している。この惑星は地球よりわずかに大きく、地球が太陽に近いよりもはるかに恒星に近いため、GJ1252bは人を寄せ付けないだけでなく、強烈に高温になっている。
GJ1252bに大気がないことを明らかにするため、二次食の際に惑星から放射される赤外線を測定した。
日中の気温は華氏2,242度にも達し、金や銀、銅が溶けてしまうほど高温であることが明らかになった。この熱さと地表の気圧の低さから、研究者は大気が存在しないと考えた。
＜関連情報＞
- https://news.ucr.edu/articles/2022/10/21/discovery-could-dramatically-narrow-search-space-creatures
- https://iopscience.iop.org/article/10.3847/2041-8213/ac886b
GJ1252b：大気を持たない高温の地球型スーパーアース GJ 1252b: A Hot Terrestrial Super-Earth with No Atmosphere
Ian J. M. Crossfield, Matej Malik, Michelle L. Hill, Stephen R. Kane, Bradford Foley, Alex S. Polanski, David Coria, Jonathan Brande, Yanzhe Zhang, Katherine Wienke, Laura Kreidberg, Nicolas B. Cowan, Diana Dragomir, Varoujan Gorjian, Thomas Mikal-Evans, Björn Benneke, Jessie L. Christiansen, Drake Deming, and Farisa Y. Morales
The Astrophysical Journal Letters Published 2022 September 23
DOI:https://doi.org/10.3847/2041-8213/ac886b
Abstract
In recent years, the discovery of increasing numbers of rocky, terrestrial exoplanets orbiting nearby stars has drawn increased attention to the possibility of studying these planets’ atmospheric and surface properties. This is especially true for planets orbiting M dwarfs, whose properties can best be studied with existing observatories. In particular, the minerological composition of these planets and the extent to which they can retain their atmospheres in the face of intense stellar irradiation both remain unresolved. Here, we report the detection of the secondary eclipse of the terrestrial exoplanet GJ 1252b, obtained via 10 eclipse observations using the Spitzer Space Telescope’s IRAC2 4.5 μm channel. We measure an eclipse depth of ppm, corresponding to a dayside brightness temperature of K. This measurement is consistent with the prediction for a bare rock surface. Comparing the eclipse measurement to a large suite of simulated planetary spectra indicates that GJ 1252b has a surface pressure of ≲10 bar, i.e., substantially thinner than the atmosphere of Venus. Assuming energy-limited escape, even a 100 bar atmosphere would be lost in <1 Myr, far shorter than our gyrochronological age estimate of 3.9 ± 0.4 Gyr. The expected mass loss could be overcome by mantle outgassing, but only if the mantle’s carbon content were >7% by mass—over two orders of magnitude greater than that found in Earth. We therefore conclude that GJ 1252b has no significant atmosphere. Model spectra with granitoid or feldspathic surface composition, but with no atmosphere, are disfavored at >2σ. The eclipse occurs just +1.4 minutes after orbital phase 0.5, indicating = +0.0025 , consistent with a circular orbit. Tidal heating is therefore likely to be negligible with regard to GJ 1252b’s global energy budget. Finally, we also analyze additional, unpublished TESS transit photometry of GJ 1252b, which improves the precision of the transit ephemeris by a factor of 10, provides a more precise planetary radius of 1.180 ± 0.078 R⊕, and rules out any transit-timing variations with amplitudes ≳1 minute.
