Can the "super-Earth" outside the solar system become our new home?

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.

Recently, astronomers used the James Webb Space Telescope (JWST) to detect for the first time an atmosphere rich in carbon dioxide or carbon monoxide on a rocky planet outside the solar system. Although the planet may be covered by a magma ocean and cannot support life, its study can enhance our understanding of the early history of the Earth. The relevant paper was published in Nature on May 8, 2024.

The James Webb Space Telescope (JWST) is a space telescope developed jointly by the United States, Europe, and Canada that mainly observes in infrared light. It took more than 20 years and cost more than $10 billion. It is the most expensive, most powerful, and largest space telescope ever built. The telescope will be launched in 2021 and is designed to provide higher infrared resolution and sensitivity to observe deeper into the universe.

55Cancri e Artistic image very close to the star

Credit: Mark Garlick/Science Photo Library

However, 55 Cancri e was not discovered by JWST. It was the fourth planet discovered around the star 55 Cancri (named Copernicus by the International Astronomical Union in 2015) 20 years ago (2004). Currently, five planets have been discovered around this star (namely, b, c, d, e, and f). These planets were discovered through the radial velocity method (i.e., by detecting the radial velocity changes caused by the mutual rotation of the star and the planet).

The existence of this planet was further confirmed by the discovery of 55 Cancri e's transit in 2011. 55 Cancri e orbits a star similar to the Sun and is a super-Earth with a radius about twice that of Earth, a mass more than eight times that of Earth, and an atmosphere that is a few percent of Earth's radius.

For the first time, humans have detected the atmospheric composition of an Earth-like planet outside the solar system

The contribution of JWST is actually the discovery that the surface of this planet contains an atmosphere rich in carbon and oxygen elements, which means a new milestone. This is the first time that humans have detected the atmospheric composition of an extrasolar planet and determined it to be a "secondary atmosphere."

Image source: veer gallery

The so-called "secondary atmosphere" is relative to the original atmosphere. The original atmosphere of a planet refers to the atmosphere that was absorbed by gravity in the early stage of the planet's formation, mainly hydrogen and helium (because these two are the most abundant in the universe). However, hydrogen and helium are very light. For ordinary rocky terrestrial planets such as the Earth, their gravity is small and not enough to completely bind these light gases, so these original light gases will gradually escape back into space.

The secondary atmosphere is the gas released from the interior of the planet later, such as through volcanic eruptions. These gases mainly contain "heavy" elements, such as carbon and oxygen. Heavy gases are not easy to escape and gradually accumulate and are preserved on the surface of the planet. The atmosphere on our earth gradually evolved from the secondary atmosphere.

Will this "super-Earth" be humanity's next home?

Current observation technology cannot directly image such a small planet to see its appearance. However, astronomers can infer what it looks like through the properties of the planet.

55 Cancri e can be simply understood as an enlarged version of the Earth. It has similarities with the Earth, such as the average density is not much different, and both are rocky planets. However, its orbital period is only 0.74 days, and its distance from its host star is only one-sixtieth of the distance between the Earth and the Sun. Therefore, this star is "baked" by its host star and is very hot. The temperature on the side facing the star exceeds 3000 degrees, and the surface is all molten magma. It should not be able to give birth to so much life like the Earth.

Image source: veer gallery

In the future, as detection technology continues to develop, humans will be able to detect the atmospheres of more Earth-like planets. By studying their atmospheric composition and comparing them with the planets in our solar system, this will help us better understand the formation and evolution of planets and their atmospheres.

We can't go back to the early days of Earth to learn about its history, but if we can detect the atmospheres of many terrestrial planets at different ages, we can use them as mirrors to see what Earth's past and even future will look like.