Scientists have discovered another habitable planet, which is expected to become the next "Earth"?

Produced by: Science Popularization China

Author: Xie Jiwei (Nanjing University)

Producer: China Science Expo

Editor's note: In order to expand the boundaries of cognition, the China Science Popularization Frontier Science Project has launched a series of articles on the "Unknown Realm", which provides an overview of the exploration results that break through the limits in deep space, deep earth, deep sea and other fields. Let us embark on a journey of scientific discovery and get to know the amazing world.

Among the issues that the public is most concerned about in the universe, the exploration of the "second Earth" is undoubtedly at the top. Astronomers have discovered more than 5,000 exoplanets in the vast universe, but the number of Earth-like planets in the habitable zone is only about dozens. Therefore, every discovery of an Earth-like planet in the habitable zone is an exciting thing. It keeps telling us that although the Earth is a very special planet, it may not be alone in the universe.

Recently, an international team of astronomers discovered another exoplanet very close to Earth that may be suitable for human habitation - Wolf 1069 b. This exoplanet orbits the star Wolf 1069, 31 light-years away from Earth.

Earth-like planets and habitable planets

Terrestrial planets are planets similar to the Earth. The similarity here generally refers to the similarity of the structure to the Earth, that is, the center is a metal core ball composed mainly of iron, and the core ball is surrounded by a mantle and crust composed of various rock materials.

Currently, Mercury, Venus, Earth, and Mars in our solar system are all considered terrestrial planets. Humans have launched probes to conduct detailed exploration of terrestrial planets in the solar system. Through these probes, humans can understand the topography, internal material composition, magnetic field, climate environment, etc. of terrestrial planets, and explore the history of the formation of terrestrial planets and the origin of life.

Image source: veer gallery

The exploration of terrestrial planets outside the solar system has just begun, and humans have already discovered a number of terrestrial planets outside the solar system. In the near future, this aspect will make rapid progress, and the number of terrestrial planets discovered will soon reach hundreds, thousands, or even tens of thousands. It is also possible to accurately characterize the atmospheric properties of some of the targets that are relatively close and easy to observe, and it is expected that characteristic signals of the existence of life will be found. The overall statistical and comparative study of them will also give us a new understanding of the formation of terrestrial planets.

We humans live on Earth, which is the only planet we know of that is suitable for life. Therefore, the discovery of Earth-like planets can be said to be the first step in finding habitable planets.

Image source: veer gallery

The habitability of a planet requires many conditions to be met . The presence of liquid water is only one of the basic conditions. There are also some other conditions, such as it is generally a rocky planet (the terrestrial planet mentioned earlier), and it must have a certain thickness of atmosphere to protect it from the bombardment of high-energy particles and asteroids in the universe, and maintain the stability of the surface temperature. For example, it may also need a certain magnetic field to protect it from the harm of stellar winds and cosmic rays, and it also needs a stable host star like the sun. If the star has frequent and violent eruptions, it will threaten the habitability of the planet.

In fact, it is difficult to say what makes a planet habitable. We can only select some candidate celestial bodies that may be habitable planets based on the basic conditions mentioned above. Because even if these conditions are met, the planet may not be habitable, because there may be conditions that we don’t know about. In the final analysis, our understanding of the origin and evolution of life is not thorough enough.

Focus on Wolf 1069 b

We discovered Wolf 1069 b through the radial velocity method. The principle of this method is very simple. It is said that planets revolve around the sun, but in fact, all motions are relative. Stars also revolve relative to planets, and the periods of the two are the same. Therefore, by monitoring the changes in the radial velocity of stars relative to the observer, we can know the surrounding planets.

Image source: veer gallery

Wolf 1069 b is 31 light-years from Earth, which is very close on an astronomical scale. Currently, the closest known star to our solar system is Proxima Centauri, which is about 4 light-years away. Wolf 1069 b is currently the sixth closest Earth-like planet in the habitable zone, with the most distant ones being thousands of light-years away.

The mass of Wolf 1069 b found this time is about 1-2 times the mass of the Earth. Judging from the general mass-radius relationship, it is likely to be a rocky planet about the same size as the Earth. It meets the basic requirements for a habitable planet. First, it is an Earth-like planet based on its mass; second, it is in the "habitable zone", with a moderate distance from its host star and a surface temperature that allows the existence of liquid water.

Wolf 1069 b is a tidally locked planet, similar to the Moon, so its rotation and revolution are synchronized. Because the Moon is tidally locked, one side is always facing us. Wolf 1069 b also has one side facing its star, so it is always daytime on this side and nighttime on the other side.

Image source: veer gallery

The star that Wolf 1069 b orbits is a red dwarf. The habitable zone of a red dwarf is very close to the star and is prone to tidal locking, so it may be more suitable for habitation only in the transition zone between day and night mentioned above. In addition, red dwarfs are generally more active than the sun, so we need to find some "abnormal" and "quiet" red dwarfs. Earth-like planets in the habitable zone around them are more suitable for habitation.

Exploring signs of life and searching for humanity’s “second home”

It can be preliminarily determined that Wolf 1069 b has some of the conditions required to be a habitable planet, but it is not yet possible to determine whether there are signs of life on this planet.

First of all, the exoplanets are very far away from us. We cannot directly see the surface of the planets to see signs of life. We can only speculate through some very indirect means, such as by detecting the composition of the planet's atmosphere (this is already difficult, especially for terrestrial planets, whose atmospheres are too thin and require high observation instruments and technology). Even if we can analyze the composition of the planet's atmosphere, there is still a more difficult thing, which is what is the signal of life signs. We can speculate based on the experience of life on Earth, but there may be other forms of life, whose signs of life are different from what we know, so there are many uncertainties.

Image source: veer gallery

In the future, it is expected that we will continue to monitor whether there are other planets in the system where Wolf 1069 b is located, and obtain more orbital parameter information of this planet to reveal the possible formation history of this planet.

The discovery of Wolf 1069 b is a good addition to the known candidate star library of habitable planets, marking another step forward in the search for habitable stars. It is an encouragement. Its discovery also inspires us to search for habitable planets not only around stars like our sun, but also around different types of stars (such as red dwarfs like Wolf 1069).