How much do we know about habitable exoplanets?

 While searching for exoplanets, scientists were surprised to find a planet similar to the Earth. TOI 700 d is the newest member of the Earth-like planet. The planet was discovered by NASA's Transiting Exoplanet Survey Satellite (TESS). It is one of three celestial bodies in a distant solar system. This planet is not like the surrounding celestial bodies and most planets discovered by scientists. Its size is similar to that of the Earth. In addition, the distance it revolves around the star allows liquid water on its surface.

　　 As a planet the size of the Earth, TOI 700 d is also located in the habitable zone of its orbiting star, which is very important for scientists. "We have always wanted to understand a question. Can life form on planets orbiting very small stars? The discovery of TOI 700 d gave us a big step towards this goal," said at the Harvard-Smithsonian Center for Astrophysics Joseph Rodriguez, an astronomer working at (Harvard-Smithsonian Center for Astrophysics), said, “We are far from answering this question. It may take decades or even longer to answer this question. However, we are moving towards Moving forward in the direction of solving one of the most important problems in the scientific community."

　　 However, despite the excitement, scientists do not fully understand TOI 700 d. They know that the star of TOI 700 d is a red dwarf, which is milder than some other stars. Active stars throw electromagnetic storms and high-energy charged particles on surrounding planets, making life impossible on these planets.

"This star is extremely quiet," said Emily Gilbert, a graduate student in astronomy at the University of Chicago. "We used TESS to observe it for 11 months and didn't see any flares. We guessed that because the star is a bit old, so It has become more peaceful in the course of its life."

　　 So far, scientists have discovered three planets around this quiet star, namely TOI 700 b, TOI 700c and TOI 700 d. Among them, the orbits of TOI 700c and TOI 700 d are too close to the stars and are not suitable for life. But the orbit of the third planet is in the habitable zone of the star, where the temperature can keep the water on the planet's surface liquid. Gilbert said: "TOI 700 d is actually deeper than the earth in the habitable zone. The earth itself is barely habitable."

　　 But scientists have also determined an undesirable phenomenon: this planet is locked by the tide of the star, one side of it always faces the star, it is daytime, and the other side is always night.

　　 But apart from these, scientists don’t understand other features of this planet. Scientists studying TOI 700 d particularly want to know a key data: its quality. This data will explain the extent to which this planet is a rocky planet like Earth, rather than a gas-filled planet like Neptune.

　　 Since the quality data of TOI 700 d is difficult to measure, the scientists first announced their findings(hermes outlet). "There are instruments to make measurements," Rodriguez said, "but there are few such instruments. This process may last for several years and requires hundreds of observations by multiple research teams." Scientists do not yet know whether this planet is There is an atmosphere, which is also an important clue to the possibility of life. Gilbert said, "If this planet is just a mass of rocks, no life can live there." Unfortunately, answering this question is more difficult than measuring the mass of the planet.

　　 So until now, scientists have assumed that TOI 700 d is made of rock, and tried to use the model to establish the connection between their understanding of the planet and the formation of life. Gabrielle Engelmann-Suissa, Visiting Research Assistant at the University Space Research Association of Goddard Space Flight Center, said, “The construction model helps us understand the vitality of this planet and how much it can maintain a livable surface temperature under various conditions. ."

　　 In short, Engelmann-Suissa and his colleagues ran 20 different models. Each model adopts a different combination of surface features, such as whether the planet is covered by land or by water; is the atmosphere like today's earth, or is it like ancient earth or Mars?

　　 Engelmann-Suissa and colleagues don’t know whether any of these models are more in line with the actual situation of TOI 700 d. She said: "When you simulate various types of planets, it sounds a bit confusing." However, the meaning of simulation is not to accidentally discover situations that match the facts. On the contrary, the meaning of this is to be able to understand the various possibilities and understand whether they can be distinguished by scientific instruments.

　　 On the one hand, the model of TOI 700 d has a certain hope to evolve into life. "No model has a runaway greenhouse effect," Engelmann-Suissa said. "In the models we studied, none of the oceans evaporated, which is a good sign." She added that the global average of these models The temperature is quite huge, but it has not exceeded the limit that scientists envisioned, especially those tenacious life can resist.

　　In the model with the highest average temperature, the average surface temperature displayed is about 91°C. "For us humans, this temperature is too high," Engelmann-Suissa said, "but it is not rigorous to say that there is no life there."

　　 The second purpose of simulation is to better understand how to use instruments to understand the world and provide a rigorous assessment of TOI 700 d. The instruments currently available to scientists cannot distinguish all these planets by their potential characteristics. NASA's next-generation major telescope, the James Webb Space Telescope, cannot be distinguished either, and the conceptual design of most future telescopes will rely on similar equipment.

　　 Engelmann-Suissa said, “This is a big problem in the field of astronomy, and the hope of observing these planets is somewhat slim. We really need to experiment with the probe to figure out how to get the signal accuracy.”

　　 However, scientists know for sure that starting from this summer, TESS will once again aim at TOI 700 d planet, which may uncover new puzzles that scientists try to solve. "Perhaps, we will find that we do not understand the structure of that system, and maybe there are some other planets," Rodriguez said. "In addition, although it is a rare case, we will also find that the system begins to become similar Our solar system."

　　 "But we don't know yet. I think this is really interesting," Rodriguez said. "We will get more data. Right now, we are just beginning to understand what that celestial system is like."