Looking for livable exoplanets is looking for "Earth 2.0"?

There may be hundreds of billions of exoplanets in the Milky Way. ESO/M. Kornmesser

Author Lin Wenjie

There are about billions of stars in our galaxy. If every star has at least one planet, then the Milky Way will have10 billion to 400 billion planets. Are there any planets like the earth? Is there intelligent life like human beings on it? This has always been the goal of astronomers.

For a long time, our understanding of planets has been limited to the eight planets in the solar system. It was not until 1995 that astronomers first discovered the exoplanet pegasus 5 1b, which revolved around a sun-like star. However, in the next 20 years, the total number of exoplanets discovered by human beings exceeded 4,000, among which the super earth similar to Earth was particularly noticeable.

On March 5th, a paper in the journal Science reported that astronomers discovered another super earth named Gliese 486b. It is rare that this super earth still retains its atmospheric composition, which makes it less hot and becomes an ideal place for astronomers to study the planetary atmosphere.

Super earth with atmosphere

Gliese 486b is a rocky planet, its mass is 2.8 times that of the earth, and its radius is 1.3 1 times that of the earth, which is equivalent to a super earth.

Its "sun" is a red dwarf. Gliese 486b surrounds this red dwarf star, and its revolution radius is only 2.5 million kilometers. It is one of the exoplanets whose orbits are closest to the stars. Accordingly, its period of revolution is very short, only 1.5 days, which is a short-period version of the super earth, which is very special.

The closer to the star, the more serious the disturbance of the nearest matter to the planet, and the flare that broke out on the red dwarf will also directly act on the planet, making the surface environment of the planet terrible.

In such a bad situation, the atmospheric material of the planet is often stripped and lost, and becomes like a comet, dragging the lost atmospheric tail while rotating. However, Gliese 486b still retains the atmosphere composed of hydrogen and helium, so it is of great research value.

In the past few years, scientists have discovered mostly short-period gaseous planets, while Gliese 486b is a rocky planet like Earth. With its help, we can study how the Earth-like planets lose atmospheric matter in orbits so close to the stars, and what will happen to the planet's surface after stripping off the atmosphere.

In fact, this is the future fate of the earth. As the sun enters the red giant stage and expands outward, the earth will passively become such a short-period planet. So seeing Gliese 486b is like seeing a preview of the fate of the earth.

There may be volcanic lava flowing on the surface of Gliese 486b. Rendering area

Will it be a livable planet?

The surface temperature of Gliese 486b planet is estimated to be 430°C, which is not too hot, but lower than that of Venus. But it is still too hot for our understanding of life forms.

Compared with other short-period planets, the surface temperature of Gliese 486b is actually low. For example, the surface temperature of the exoplanet Cancer 55E, which is 4 1 light-year away from the Earth, reaches 2300℃ ... The low temperature of Gliese 486b may be influenced by the atmosphere, which is a key reason why Gliese 486b is worth studying.

In fact, almost all the currently registered short-period exoplanets are uninhabitable.

First of all, their orbital radius is too small, which will be affected by the depth of the main star. The most direct interference is the extremely high surface temperature.

Secondly, most short-period exoplanets may be tidal locking, with one side always facing the star, which is extremely hot, and the other side facing away from the star, which is always in darkness, just as the moon only faces the earth. As a result, not only the temperature is extreme, but also the lighting conditions are extreme, which is extremely unfavorable to the atmospheric circulation of the whole planet.

Third, the flare of a star will directly affect the planet, producing strong electromagnetic radiation on the side facing the star, which is extremely unfavorable for us to understand the life form.

It can be seen that the super earth Gliese 486b is actually uninhabitable. So what does livable mean? Maybe it's not just finding another earth.

Kepler 452b is also a super earth and one of the most earth-like planets. But the super earth is not necessarily suitable for human habitation. NASA/Ames /JPL- California Institute of Technology

Livable is looking for "Earth 2.0"?

When it comes to livable exoplanets, we usually imagine the "earth model".

Meet the following basic conditions: first, the surface temperature of the star is suitable, the life span is long, and there is enough time for life to evolve. Secondly, the planet needs to be in a livable zone around the star, neither too hot nor too cold, allowing water to exist in liquid form. In addition, the orbit of the planet should be stable enough not to be hit by celestial bodies for no reason, leading to the extinction of life.

The basic thing is to find "Earth 2.0" one by one. After all, what we know at present is that planets like the earth can indeed give birth to life. If human beings want to find a home for future interstellar immigrants, they should probably keep looking for it according to such standards. However, if we want to pass the screening of layer-by-layer conditions, we will find that there are very few livable exoplanets, and only 90% of the stars in the Milky Way are eliminated by star types.

But we can step back and think about it. Does a livable planet have to be like the earth? For example, we usually think that water is an indispensable part of life, but does life really need water? Must they be carbon-based life? Even, does the earth need solar radiation to get energy?

Maybe not. In the extreme environment of the universe, there may be life forms beyond our understanding, just as scientists once discovered a new vibrant life world in the deep-sea hot springs of the earth.

With the development of exoplanet detection technology, there will be more and more livable planets and super-earths in the future. Humans may need to expand their imagination and understand that livable planets may not be able to breed life, and planets that are not in the livable zone may also be inhabited by strange alien life.

After all, among the billions of stars in the Milky Way, some stars are like the sun, with multiple planets orbiting them, and sometimes a planet orbiting two stars-on these planets, even shadows will not be lonely.

So, who knows how vast the possibility of the universe is?

The exoplanet Kepler-16b revolves around two stars, where the shadow will not be lonely.

Editor Chen Tianzhen

Reference link

/a/ Super Earth is an astronomer's dream of atmospheric research/

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