Tomonori Toya, Professor, Department of Astronomy

Key points of the presentation

• We have shown that it is possible to search for life outside the solar system by capturing micron-sized particles falling to Earth from outside the solar system.
• This is a new and unique method to search for extraterrestrial life, as it enables us to obtain direct samples of traces of primitive life that originated outside the solar system.
• We can explore how many life-forming planets there are in our own galaxy. It is hoped that the technology to achieve this in the future will be investigated.

Summary of the presentation

Professor Tomonori Toya of Graduate School of Science, The University of Tokyo proposed a new method to search for life outside our solar system. In a planetary system around a star outside the solar system, an asteroid collides with a life-supporting planet, and rocks on the planet's surface are ejected into space as fine particles. They leave the planetary system and travel through interstellar space ^{(Note 1)}, with some eventually reaching the Earth in our solar system. In this study, it was estimated that about 100,000 such particles are falling to Earth each year. In the future, by capturing these particles, we may be able to directly obtain traces of life that originated in planetary systems far from our solar system. This could answer the question of how many of the galaxy's many stars have life-supporting planets. It is hoped that technical studies will be conducted to make such exploration feasible in the future.

Contents of presentation

Background of the research
Does life exist outside the Earth? This is one of the ultimate questions for human beings beyond the framework of science. Currently, there are several approaches to the search for extraterrestrial life. One is to send probes to planets and satellites in the solar system to search for life and its traces. However, there is no guarantee that there is extraterrestrial life in our solar system. If there is no life, the only way to find it is to search for life in planetary systems of stars other than the sun. In astronomy, the next generation of space telescopes may be able to observe exoplanets orbiting stars close to the Sun and search for life signatures, such as oxygen in the atmosphere. However, it is difficult to prove the existence of life outside the solar system without anyone's doubt with this method. With only indirect observations from afar, without going to the site, it is difficult to determine whether the supposed traces of life are really caused by life or whether they were generated nonbiologically. Another is the search for radio signals emitted by extraterrestrial civilizations, known as SETI. If such a signal is captured, it could provide solid proof, but the probability of evolution from primitive life to civilization may be extremely low. New methods to search for primitive extrasolar life are desirable.

<Research Contents
In this study, we focused on the impact of giant meteorites on Earth-like planets outside our solar system, which eject rocks and particles of various sizes into space. It is certain that this happens frequently, and in fact, some of the meteorites collected on Earth came from Mars, and these are also thought to have been thrown out when an asteroid hit Mars and eventually fell to Earth. When an asteroid hit the earth about 66 million years ago and caused the extinction of the dinosaurs, a large amount of such rocks and particles must have been scattered from the earth into space. The possibility of living life traveling between planets or stars on such rocks has been discussed as the panspermia theory, which seeks the origin of life in space. However, relatively large rocks (about 1 kilogram or more) are needed to protect living life from cosmic rays and radiation in space. The likelihood of such a rock traveling beyond interstellar space and arriving with living life in a planetary system on another star, let alone traveling within the solar system, is considered extremely unlikely.

The inspiration for this research was the idea that "if you want traces of life, not living life, then smaller particles are fine. Such particles are much larger in number, so the number of particles coming to Earth must be large, too. This was the idea. For this purpose, particles of about one micron (one-thousandth of a millimeter) in size are promising. This is the size that can contain dead microorganisms and fossils, and such small particles can easily escape from planetary systems and fly out into interstellar space because the pressure from the sun's radiation is about the same as that of gravity.

In this study, we have examined the various processes by which particles escape from terrestrial planets associated with numerous stars in our galaxy, travel and are lost in interstellar space, and finally reach the Earth in our solar system, and have come up with the first estimate of the number of such particles.

The result is that 100,000 particles fall to Earth each year. If life is common on many of the planets in our galaxy, and if the surface of a planet like Earth is teeming with life, these particles should contain fossils of extrasolar microorganisms and rocks and minerals derived from living organisms (limestone is an example). It is believed that such small-sized particles would be quickly slowed down upon entering the Earth's atmosphere and would fall to the ground without becoming too hot. Particles imprinted with traces of life outside our solar system may be falling daily from above our heads to the ground nearby. If we can collect these particles, we can answer the question, "How many life-bearing stars are there in our galaxy? We can approach the question, "How many life-bearing stars are there in our galaxy?

Caption: Conceptual diagram of how particles containing traces of life are emitted from planetary systems outside our solar system and reach the Earth.
Please note that the following credit image is used in this illustration.
Image credit: NASA/Don Davis (imaginary meteorite impact), NASA, ESA and G. Gilmore (University of Cambridge); Processing: Gladys Kober (NASA/Catholic University of America) (background space image)

Future Prospects
The actual capture of these particles is not easy and still requires much study. The most difficult part is to sort out these particles from the huge number of particles. In the interplanetary space of our solar system ^{(Note 1),} fine particles called interplanetary dust ^{(Note 2)} are constantly drifting and falling to the earth in quantities of tens of thousands of tons per year. Searching for particles that originate outside the solar system is like looking for grains of sand in a desert. But 100,000 particles per year suggests the extremely valuable possibility that we may in the future have a direct sample of traces of life outside our solar system.

There are many possible ways to capture these particles. The most direct way is to directly capture them with a large number of detectors in space. In fact, particles of interstellar dust (Note 2) drifting in interstellar space and thought to have entered the solar system have been detected by satellites. By examining the orbits of the particles, it is possible to determine whether they came from outside the solar system. Interplanetary dust that has fallen to Earth has also been detected in Antarctic ice and in clay deposited on the bottom of the deep sea. There may be a possibility to look for particles from outside the solar system in them.

When Einstein predicted the existence of gravitational waves ^{(Note 3)} about 100 years ago, no one at the time thought they could actually be detected. Now mankind is detecting that the length of a 4 km detector has shrunk by only one thousandth of a proton due to a black hole merger that occurred a billion light-years away. First of all, it all starts with the recognition that such things exist. We look forward to further developments in science in the future.

Paper Information

Journal name	International Journal of Astrobiology
Title of paper	Solid grains ejected from terrestrial exoplanets as a probe of the abundance of life in the Milky Way
Author(s)	Tomonori TOTANI
DOI Number