Tokyo Institute of Technology (Tokyo Institute of Technology) will launch the “James Webb Space Telescope” (JWST) scheduled to be launched by NASA in October 2021 on October 7, and JAXA and ESA (European Space). By using the infrared telescope “SPICA” (Space Infrared Telescope for Cosmology and Astrophysics), which is being developed with the aim of launching around 2030, the material of a solid planet that revolves around the stars Announced that it has devised a method that makes it possible to explore.

The result is based on the research team of Ayaka Okutani, a graduate student of the Earth and Planetary Sciences Department of the University of Science (2nd year doctoral course), Associate Professor Satoshi Okuzumi, Researcher Kazumasa Ohno, and Assistant Professor Teruyuki Hirano. .. Details were published in the American astronomy magazine “The Astronomical Jounal”.

Exoplanets can be very different in composition from the planets of the solar system, and in theory there could be “carbon planets” consisting of solid carbons and carbides. Until now, attempts have been made to clarify the composition of the exoplanet from the radius and mass derived from observations, but unfortunately this method has the problem that it is not possible to narrow down the candidates for the composition of the planet to one. It was.

Therefore, the research team paid attention to the “demolition planet” that revolves near the star. Demolition planets are planets that are thought to be evaporating even their solid surfaces because they are heated by stars and are hot. It is said to be a celestial body that holds the key to elucidating the composition of exoplanets whose main component is solids.

Observations so far have revealed that demolition planets have comet-like dust tails. The dust particles are interpreted as solid vapors evaporating from exoplanets that have cooled and solidified in outer space. In other words, if the composition of this dust tail is known, it is possible to determine what kind of solid material the dismantled planet, which is the source of the dust tail, is composed of. The research team has proposed a method that combines JWST and SPICA as an observation method for locating the components of this dust tail.

  • Demolition planet passing in front of a star

    Image of a dismantled planet and a dust tail passing in front of a star (C) NASA / JPL-Caltech (Source: Tokyo Tech website)

The dust tail (solid grain) absorbs light and electromagnetic waves of different wavelengths in different proportions depending on its composition. Therefore, when observing stellar light that can be seen through the dust tail at various wavelengths, the light is strongly absorbed at one wavelength and almost transmitted at another wavelength, depending on the composition of the solid grains that form the dust tail. It happens that it comes.

The ratio of light transmission for each wavelength is called a “transmission spectrum”. For example, the transmission spectrum of visible light is the color of the dust tail as seen by the human eye. On the other hand, JWST and SPICA can capture the infrared transmission spectrum, which makes it easier to identify the composition of dust.

The research team first assumed three types of demolition planets when assuming the composition of the dust tail. Like the Earth, it is a “planet whose interior is divided into a metal core and a rock mantle,” a “coreless planet” that does not have a metal core but contains a large amount of iron in the rock, and a carbon planet. After predicting what kind of mineral composition each dust tail would produce, the theoretical calculation of the transmission spectrum of those dust tails with respect to infrared rays was performed. As a result, it was found that the difference in transmission spectrum for each candidate mineral appears significantly for each observation wavelength range of JWST and SPICA.

  • Dust tail

    An example of the transmission spectrum of a dust tail made of various minerals obtained by theoretical calculation. The light blue region represents the observation wavelength range of JWST, and the orange region represents the observation wavelength range of SPICA (Source: Tokyo Institute of Technology website).

For example, magnesium-rich minerals such as the Earth’s mantle and iron-rich minerals, which are candidate minerals for the dust tails of coreless planets, show significantly different spectra in the SPICA observation wavelength band. On the other hand, silicon carbide, which is a mineral candidate for the dust tail of carbon planets, cannot be specified in the observation wavelength band of Spica, but it can be specified because it absorbs infrared rays of a specific wavelength in the observation wavelength band of JWST. It is said that In this way, by combining JWST and SPICA, it was shown that it is possible to systematically narrow down the candidates for the mineral composition of the dust tail.

In addition, the research team also conducted observation simulations based on the performance of the mid-infrared observation equipment to be mounted on the JWST and SPICA telescopes in order to investigate the observation conditions that make it possible to identify minerals. As a result, it was confirmed that the mineral composition of the dust tail can be identified with sufficient observation accuracy for a dismantled planet that is within about 3000 light-years from the earth and has a dust tail of a typical size. I said that.

NASA’s space telescope “TESS” (Transiting Exoplanet Survey Satellite), which is currently in its third year of extended missions to search for exoplanets, and the space that ESA plans to launch in 2026. If a newly dismantled planet is discovered within 300 light years, which is 1/10 of the distance of the simulation result, by the telescope “PLATO” (PLAnetary Transits and Oscillation of stars), the observation method proposed this time will be applied. It is said to be a good target.

If the composition of the dismantled planet itself is revealed by infrared observations in space that will be realized in the near future, including JWST and SPICA, humankind’s understanding of the composition of exoplanets, which is still unknown, should be greatly advanced. Among them, the types of minerals contained in the planet are thought to reflect where and how the planet was formed, and the research team is approaching the formation process of the dismantled planet by observing the dust tail. I’m expecting it.