A research team led by researchers at the Astrobiology Center and the University of Tokyo has observed low-temperature “ultra-short-period planets” with an orbital period of less than one day by observing with the Subaru Telescope’s near-infrared spectrometer IRD. It was discovered around a star and revealed that its internal composition consists mainly of iron and rock. The planets (TOI-1634b and TOI-1685b) found around the two low-temperature stars are both equivalent to Super-Earth (Note 1), which is about 1.5-2 times the size of the Earth, and in particular, TOI-1634b It is one of the terrestrial planets with the largest radius (1.8 Earth radius) and mass (10 Earth mass) among the ultra-short-period planets found so far. Planets of this size are on the border between rock and gas giants, especially around low-temperature stars, so how many planets have a “one year” less than the length of a day on Earth? It can be said that the most valuable celestial body was discovered in investigating whether it was formed.
Observations have revealed that about 1% of extrasolar planets (exoplanets) are planets with an orbital period of less than one day (ultra-short-period planets). It is thought that ultra-short-period planets formed in outer orbits may have moved to inner orbits due to interactions with other planets, etc., in order to understand the formation of various planets. , A rare and important celestial body.
Most of the ultra-short-period planets observed so far are small planets with a radius of 1.5 times or less that of the Earth, and it is known that their internal composition is similar to that of the Earth, which is mainly composed of iron and rocks. However, most of the ultra-short-period planets scrutinized in this way are known only around sun-like stars (solar stars), and there are only a few observations around low-temperature, small-mass stars. is. Low-temperature stars are known to have multiple small planets, so ultra-short-period planets may also be present. A closer look at the frequency and characteristics of ultrashort-period planets around low-temperature stars is expected to give a general understanding of the origins of ultra-short-period planets.
The research team focused on two low-temperature stars TOI-1634 and TOI-1685 with transit planet candidates (Note 2) detected by NASA’s Transiting Exoplanet Exploration Satellite “TESS”. .. The mass of these stars is only about half that of the Sun. Independent analysis of TESS data and follow-up observation of transit using the MuSCAT series of multicolor simultaneous imaging cameras (Note 3), followed by spectroscopic observation using the Subaru Telescope’s infrared spectroscope IRD (InfraRed Doppler). Did. The IRD is a spectroscope that accurately measures the radial velocity of a star (radial velocity), and is a unique observation device optimized for observing low-temperature stars that appear brighter with infrared rays than visible light.
As a result of detailed analysis of the radial velocities observed by IRD, the actual ultra-short-period planets around TOI-1634 and TOI-1685 were 0.989 days (TOI-1634b) and 0.669 days (TOI-1685b), respectively. It was confirmed that it revolves in the cycle of. Furthermore, from the amplitude of the change in radial velocity, it became clear that TOI-1634b and TOI-1685b have about 10 times and about 3.4 times the mass of the earth, respectively (Note 4). When the composition of the planet was theoretically estimated based on this planet mass and the planet radius (TOI-1634b is about 1.8 earth radius, TOI-1685b is about 1.5 earth radius) obtained from transit observation, which planet It was found that, like the earth, has an internal composition mainly centered on iron and rocks (Fig. 2). Two planetary systems have been discovered in which Super-Earth, which has a composition similar to that of the Earth, revolves in the immediate vicinity of low-temperature, small-mass stars.
TOI-1634b is one of the planets with the largest radius and mass among the ultra-short-period planets confirmed to have an internal composition similar to that of the Earth, and such planets are around stars that are much lighter than the Sun. It is very interesting to find in. From the “mass-radius” relationship (Fig. 2), it was also found that there is no thick hydrogen atmosphere on both planets. On both planets, where no protoplanetary atmosphere of gas from the protoplanetary disk is left, a secondary atmosphere of gas released by the planets may be formed. It is also an interesting observation target for studying how the atmosphere of terrestrial planets that orbit the immediate vicinity of stars evolves.
Both planetary systems are located relatively close to the Earth about 100 light-years, and are particularly bright among low-temperature stars with ultra-short-period planets, making them promising observation candidates for next-generation telescopes. The lead author of the paper, Assistant Professor Teruyuki Hirano (National Institute of Natural Sciences, Astrobiology Center / National Institute of Natural Sciences, Hawaii Observatory) said, “In the future, we will observe the planetary system found in this research with the James Webb Space Telescope (JWST). By investigating the planetary atmosphere and detailed orbits, it is expected that the origin of the still mysterious ultra-short-period planets will be elucidated. Also, the planetary candidate celestial bodies identified by TESS will be intensively tracked by IRD. The observing project is still underway and many unique planets should be identified in the IRD in a year or two, “he said.
This research result was published in the American astronomy journal “Astronomical Journal” (September 23, 2021) (Hirano et al. ” Two Bright M Dwarfs Hosting Ultra-Short-Period Super-Earths with Earth” -like Compositions “).
(Note 1) “Super Earth” is a planet larger than the Earth, and refers to an exoplanet whose mass is about 10 times or less that of the Earth and whose diameter is about 2 times or less that of the Earth. Since there are no planets of such weight and size in the solar system, observations of exoplanets have revealed that such planets exist for the first time.
(Note 2) “Transit” is a phenomenon in which a star appears to be dark periodically because the planet passes in front of the star. The exoplanet system in which transit is observed is called the transit planetary system. In transit exploration such as TESS, many transit-like dimmings are detected by large-scale photometric monitor observations, including false detections by “eclipsing binaries”. It is confirmed that the “transit planet candidate” detected by TESS is a real transit planet for the first time by performing follow-up observations using other telescopes.
(Note 3) Multicolor simultaneous imaging cameras mounted on the 188 cm telescope in Okayama Prefecture, the 1.52 m telescope at the Teide Observatory in Tenerife, Spain, and the 2 m telescope at the Haleakala Observatory in Maui, USA, MuSCAT, MuSCAT2 , MuSCAT3 was used for follow-up observation of transit. For all planets, this follow-up observation accurately determined parameters such as the orbital period and planetary radius that were tentatively obtained by TESS.
(Note 4) If there are planets around the star, the star will fluctuate slightly due to the influence of the planet’s gravity. The radial velocity method captures this fluctuation as a periodic change in the radial velocity of a star. The larger the mass of the planet, the larger the amplitude of the change in radial velocity. The masses of the two planets found were determined by follow-up observations by the IRD.
Featured image: An image illustration comparing the sizes of the terrestrial planets discovered in this study. TOI-1685b is 1.5 times the diameter of the earth and TOI-1684b is 1.8 times the diameter. Both planets are surrounded by stars that are cooler than the Sun, so they are illuminated by reddish light. (Credit: National Institute of Natural Sciences Astrobiology Center)
Provided by Subaru Telescope