Super Earth: Strange New Worlds

Earlier, scientists naively believed that if we discovered other planets orbiting stars, these new planets would be similar to the planets in our solar system.

But we were wrong. The universe is always unpredictable. There are more than 1,900 exoplanets discovered so far (the number is still increasing), many of which are very different from the planets in our solar system. For example, in 2011, a planet orbiting two stars was discovered. It is like the planet Tatooine orbiting the double star in the science fiction movie "Star Wars". On this planet, the protagonist Luke can gaze at "two suns setting at the same time." New discoveries in astronomy turn science fiction stories into tangible reality.

Looking for “living planets”

But perhaps the biggest surprise for us from exoplanets is “super Earth”, also known as “super Earth-like planet”. This type of planet broadly refers to planets whose environment may be similar to that of the Earth, but whose mass is usually 1 to 10 times that of the Earth.

In 1992, astronomers discovered exoplanets for the first time, and discovered two at once. Each of them is almost three times the mass of the Earth. Generally speaking, planets orbit stars. , just like our Earth orbits the sun, but these two exoplanets are particularly unusual because they orbit a pulsar called PSR B1257 12 (located in the constellation Virgo, about 980 light-years away from the Earth).

This discovery stunned researchers because astronomers at the time generally believed that planets could only exist around main sequence stars, but the first exoplanets observed were found around pulsars. , why is there such a strange thing?

It wasn’t until 1995 that astronomers discovered a so-called “hot Jupiter” (a Jupiter-like planet orbiting very close to its parent star) around a typical sun-like star. One breath. The discovery of this planet inspired astronomers to start planning a "star hunting mission" to find extrasolar super-Earths.

In 2009, after the Kepler space telescope, which was specially designed to detect exoplanets, was launched, it scanned 150,000 stars within its field of view. Since then, good news of the discovery of super-Earths has continued to come. A series of pictures obtained by space telescopes are amazing: some of them are gaseous spheres, more like mini-Neptunes; some are rocky super-Earths that can be completely covered by sea water or Lava covered.

Density matters

Ecstatic planet hunters have discovered more worlds to add to the list of super-Earths. Until now, however, scientists knew nothing about these planets except their masses. No one could guess whether these strange planets were rocky (similar to Earth), gaseous (similar to Neptune), or intermediate. Between the two? Or are all the above types involved?

So the first big question about super-Earths is: What are they made of? To gain insight into a planet's basic material, astronomers need to find a "transit" - when a planet passes by its host star and blocks some of its light, allowing us to estimate the planet's size. Once astronomers know the size and mass of a planet, we can use physics learned in high school to calculate the planet's density (density is mass divided by volume). With the density, scientists can determine whether the super-Earth is a loose ball or a solid ball, a "dead world" or a possible "living world". Density is therefore important because it tells us about the characteristics of the planet.

The process of searching for the "transit" of exoplanets is not easy, but the wait finally made progress in 2009, when astronomers calculated the densities of two super-Earths. First, the spacecraft saw a "transiting" planet, later named CoRoT-7b, which was about 1.5 times the diameter of the Earth and 5 times the mass of the Earth. This means that CoRoT-7b is the first rocky exoplanet discovered, but it is so close to its parent star that its surface will definitely melt - possibly more like Mercury.

The second one is GJ 1214 b, which is the most studied super-Earth. Its diameter is about 5 times that of the Earth, but its mass is only 6.5 times that of the Earth, and its density is much smaller than the previously discovered CoRoT- 7b. The planet GJ 1214 b is 40 light-years away from the Earth. It is rich in water resources: three-quarters of the planet's surface is covered by deep oceans. This is the first planet full of liquid water discovered by humans outside the solar system, but it is accompanied by a high temperature of 190°C all year round. Therefore, GJ 1214 b is filled with hot and thick water vapor. Such an environment may not be possible. Life that is not suitable for the earth's form exists, but it is possible to discover other forms of life.

According to statistics, the number of super-Earths with known density is about 40. Generally speaking, exoplanets with a diameter less than 3.2 times the diameter of the Earth have a very high density and are likely to be rocky planets; when the diameter exceeds 3.2 times the diameter of the Earth, the density of the planet will begin to decrease, and it may be caused by Gaseous planets composed of lighter objects such as water, ice and gas.

How to reveal the secrets of planetary atmosphere

On Earth, the atmosphere is essential for animals, plants, and humans, so the atmosphere of super Earth is also the next research target for astronomers.

We mentioned before that during the "transit" process, after the light of the host star passes through the atmosphere of the exoplanet, some wavelength bands of light will be absorbed by specific atmospheric molecules. At this time, scientists can detect specific molecules in the planet's atmosphere by analyzing the spectrum. If enough data is obtained, scientists can theoretically analyze the basic components of a planet's atmosphere.

Assuming that the composition of the gas can be observed, it may provide a clue: does the super Earth have life? Or does life already exist on super-Earths? Because depending on what kind of gases are in the atmosphere, you can speculate whether certain life exists.

So far, there have been many exciting discoveries in the analysis of atmospheric gas types, including the atmospheric composition of "super Jupiters". Scientists have discovered that the huge atmosphere of "super Jupiter" is mainly composed of water vapor, carbon dioxide and methane, which is very similar to the composition of Jupiter's atmosphere. The research on super Earths is still in progress.

But space telescopes are very weak at detecting atmospheric components on rocky super-Earths, such as those found through spectral analysis of starlight collected near GJ 1214 b and another super-Earth HD 97658 b. The quantity of a particular molecule is small. The explanation given by astronomers is that the two planets may be wrapped in clouds like Venus. The thick clouds apparently block certain light from lower atmospheric molecules, making them difficult to identify.

Astronomers are still trying to unravel the characteristics of the clouds. In general, the molecular structure of exoplanet atmospheres will be the main target of the next generation telescope (Webb Space Telescope).

Three attributes to determine habitability

Whether a rocky super-Earth is suitable for life involves three planetary attributes that are critical to life: ocean Regulation, climate "conditioner", magnetic field.

In the case of the Earth, during its first few hundred million years as it cooled from its molten state, its outermost layers solidified into crust and then broke apart into plates that were trapped in a hot, dense atmosphere. Above the mantle area, there are collisions and grindings with each other, and beneath them there is a solid core. The hot gas emitted by the core constantly stirs the mantle, like a steaming hot pot. Plates of the Earth's crust may dive downward, run back into the mantle (triggering earthquakes), and then be melted away. Likewise, ocean water circulates through the mantle, and so on for billions of years. Through volcanic cracks between Earth's plates, rock and water return to the Earth's surface, and the cycle continues.

So, what will these three attributes look like on a super Earth?

First, let’s discuss the ocean. Super-Earths may be bulky blue planets that retain oceans that have been around for billions of years, possibly as good as or better than those on Earth, because the super-Earths may have plenty of water to recycle in the mantle.

The mantle cycle starts the plate movement of the planet, which will affect whether the super Earth has a livable climate over a long period of time. The key point of influence is greenhouse gases such as carbon dioxide.

Carbon dioxide is a very useful gas. On the one hand, it captures heat in the air and maintains the Earth's surface temperature. On the other hand, rocks and seawater absorb carbon dioxide from the atmosphere and store it. The reduction of carbon dioxide means less heat, and the planet will cool down. As surface materials such as rocks and seawater enter the mantle, the carbon is converted back into carbon dioxide gas and returned to the atmosphere through volcanic eruptions. The rising levels of carbon dioxide in the atmosphere can warm the planet. The final result of this cycle is: The earth can regulate its own temperature.

So, do super Earths also have this "temperature regulator"? There was a study in 2007 that showed that the super-Earth has higher internal heat and can produce faster convection, similar to the Earth's mantle circulation, but its convection is more dynamic and powerful, so compared with the Earth, it It seems easier to create plate tectonics, such "super" plates that can maintain levels of carbon dioxide in the atmosphere, meaning these planets have a more balanced climate than Earth.

However, at the same time, there is an opposite view: on a super Earth, gravity dominates, and different plates on the earth's crust will be firmly fixed by gravity, and plate drift and collision will not occur. Without plate activity, carbon dioxide would not be able to circulate internally and externally, and there would be no "temperature regulator" function. In this case, it would be difficult for life to be born. Subsequent studies supported both views, and years later, the matter remains unresolved.

In addition, the habitability of super Earth also faces a big problem: a magnetic field is needed inside the planet. Assuming that the earth does not have a magnetic field to withstand the impact of the solar wind, life would be difficult to establish even if it appeared. However, the sloshing liquid iron layer inside our earth will generate a magnetic field as a "shield" to protect life.

The interior of a super-Earth may be solid rather than layered like Earth, according to a study. Without the liquid metal layer, there is no magnetic field, and life cannot exist. However, let's not despair, as some research points to another possibility: the high heat inside the super-Earth may melt the magnesium oxide, and when this mineral liquefies and churns, it can create a magnetic field.

Obviously, we still need to better understand the inner workings of super-Earths and further study whether they are habitable.

Interstellar immigration may become a reality

At present, the super-Earths that have been discovered are just the tip of the iceberg. With the advancement of modern observation methods and the new generation of technological innovation, the speed of discovering super-Earths will increase. Faster and faster. In the future, humans may find planets that orbit stars like the sun and are suitable for human habitation. And once such a planet is discovered, interstellar immigration may become a reality.

In 2015, British scholars confirmed that the super-Earth-GJ 581 d was the first habitable exoplanet. This planet is about three times the size of the Earth. It is the first planet in the habitable zone discovered by humans outside the solar system. It is only about 22 light-years away from the Earth. It can be regarded as the "neighbor" of the Earth in the vast universe. , scientists believe it is a potential space immigration option for humans.

In addition, the "Transiting Exoplanet Survey Satellite (TESS)" will be launched by the Massachusetts Institute of Technology in 2017. It will mainly search for bright stars closer to the solar system to see if more can be discovered. There are more planets the size of the Earth. The more such planets, the greater the chance of life survival, and the greater the chance of interstellar immigration in the future.

This article is derived from an article in the 2016 issue of the magazine Big Technology* Science Mystery. Readers are welcome to follow our WeChat ID of Big Technology: hdkj1997