Rikkyo University

More than 5,000 exoplanets^*1 have been identified since the first one was discovered in 1995. They have been actively researched with many large-scale observation plans being formulated and implemented. It has been reported that some exoplanets have properties similar to Earth and could be habitable planets. Whether such planets have a warm environment like Earth and could actually harbor life is one of the biggest mysteries for humankind. Planetary atmospheres are important for maintaining a warm environment, but they are heated by XUV irradiation^*2 from stars, facilitating atmospheric escape^*3. Thus, maintaining an atmosphere and a warm environment was considered difficult.

Earth-like planets exist around low-mass, low-temperature stars (cool stars), which are abundant particularly in the vicinity of the solar system. It has been suggested that cool stars—which likely will be researched further in the future—continue emitting violent XUV radiation for several billion years. Therefore, the existence of planets that maintain a warm environment like Earth was theoretically considered difficult.

By using atmospheric simulations of Earth-like planets, this study revealed that atomic line radiative cooling^*4 is an important cooling process in the upper atmosphere that is subject to heat by violent XUV irradiation (Figure 1). This cooling works more efficiently as the temperature rises, thereby preventing the atmosphere from rising to high temperatures. As a result, the study showed, atmospheric escape, in which high-energy atmospheric particles escape the influence of planetary gravity, remained sluggish. While previous studies suggested that most of the energy absorbed in the atmosphere was used for atmospheric escape, this study demonstrated the opposite: The estimated atmospheric escape rate was a mere 1/10,000. This led to the researchers’ conclusion that the lifetime of a 1-bar atmosphere equivalent to that of Earth’s atmosphere can be extended to around 2 billion years even when subject to violent XUV irradiation—the lifetime comparable to a geological timescale (Figure 2). Such a violent XUV environment corresponds to those of early Earth and exoplanets around low-temperature stars, which suggests long-term atmospheric survival is possible even on such planets. These results provide important clues to understanding the warm environment on early Earth and point to the possible existence of other habitable planets with a warm environment. The findings are expected to stimulate further theoretical and observational studies.

*1 Exoplanet: A planet that orbits a star other than the sun. More than 5,000 exoplanets have been identified since the first was discovered in 1995 (NASA Exoplanet Archive as of September 3, 2022).

*2 XUV spectra: Photon with a wavelength of less than 100 nm and composed of X-rays and extreme-ultraviolet (XUV) radiation. High-energy XUV radiation is absorbed by gas species in the upper atmosphere, leading to photochemical reactions and heating.

*3 Atmospheric escape: The upper atmosphere ceased being in a gravitationally bound state after it was heated by XUV absorption, resulting in the loss of atmospheric gases to outer space. Currently, only light hydrogen and helium atoms escape from Earth’s atmosphere. On Earth-like planets subject to violent XUV radiation, nitrogen and oxygen atoms (their atmosphere’s main components) escape, resulting in loss of the atmosphere.

*4: Atomic line radiative cooling: A radiative process that occurs with the transition in the energy state of electrons rotating around atoms and ions. Energy levels that electrons possess are unique to each atomic or ionic species. Their energy distribution is dependent on the collisional transition that occurs when the electron collides with other gas species as well as on radiative transitions involving the absorption and emission of photons. Radiative transitions to a lower energy state by emitting photon can release energy from the atmosphere to outer space, thus cooling the atmosphere.