Are humans the only intelligent life in the universe? Is there another planet like Earth that is suitable for life? These are cosmic mysteries that people have been particularly concerned about for a long time.
Recently, an international research team led by the Yunnan Astronomical Observatory of the Chinese Academy of Sciences (hereinafter referred to as the Yunnan Astronomical Observatory) discovered a planet in the habitable zone around a sun-like star - the "super-Earth" Kepler-725c. Its mass is about 10 times that of the Earth. On June 3, the relevant research results were published in Nature Astronomy and received high praise from many reviewers.
The star Kepler-725 (center), the planet Kepler-725b (left), and the planet Kepler-725c "hidden" in the habitable zone of a sun-like star discovered using TTV inversion technology. Image courtesy of Yunnan Astronomical Observatory
New habitable "super-Earth"
According to Gu Shenghong, one of the authors of the paper and a researcher at Yunnan Astronomical Observatory, the planet orbits a G9V host star called Kepler-725. The host star has a similar spectral type to the sun, but is younger than the 4.6 billion-year-old sun, at only 1.6 billion years old, and has more intense magnetic field activity on its surface than the sun.
The planet is located in Kepler-725's habitable zone, a region suitable for the existence of liquid water, which is considered a key condition for the birth of Earth-like life. It takes about 207.5 days for the planet to orbit its host star, which is similar to the Earth's orbital period.
"A 'super-Earth' is in the habitable zone near a star like the sun, which means it is possible for carbon-based life similar to that on Earth to exist there," said Gu Shenghong. "It is about 160 million times farther away from us than the distance between the Earth and the sun."
New method to deduce "hidden" planets
Until now, this planet has not been captured by the Kepler Space Telescope and seems to be hiding in a blind spot. In this study, researchers used the transit time variation (TTV) inversion technique for the first time to successfully infer the existence of another planet in the Kepler-725 planetary system by observing the slight deviation between the time when another planet passes through the surface of the host star and the orbital period.
Sun Leilei, the first author of the paper and a young associate researcher at the Yunnan Astronomical Observatory, introduced that the TTV inversion technology is similar to observing the speed of a clock to determine whether there is an "invisible hand" quietly moving the clock hands.
In the past, scientists mainly used two methods to search for low-mass exoplanets. One is the transit method, which is to discover planets by observing the light emitted by the host star blocked by the planet; the other is the radial velocity method, which is to discover planets by detecting whether the host star is slightly swung by the planet in the line of sight. However, for planets like the Earth that are small in size and orbit far away from the host star, these two methods are difficult to work due to insufficient observation accuracy.
This time, the research team used TTV inversion technology, which does not require seeing the process of the undiscovered planet blocking the host star, nor does it need to detect slight swings of the host star in the direction of sight. It only needs to measure the transit time of another planet that resonates with the orbit of the undiscovered planet to indirectly sense the existence of the undiscovered planet.
"This is a very important result because it is the first time that a planet in the habitable zone of a Sun-like star has been discovered using TTV inversion technology," the reviewer commented.
The journal editors believe that this study proposes a complementary approach to detecting low-mass exoplanets, including Earth-like planets, in the habitable zones of Sun-like stars.
What to explore next?
The discovery marks a key step for the Chinese research team in their search for a second Earth.
Gu Shenghong said that the new approach and related research results established this time will provide new observation targets and detection technology support for China's future space astronomy missions, such as the China Manned Space Engineering Survey Space Telescope and the Earth 2.0 Project.
"The relevant research group plans to apply the TTV inversion technology to more exoplanet systems to find exoplanets 'hidden' in the habitable zones of sun-like stars and red dwarfs," said Gu Shenghong. "At the same time, we will also combine other observational methods, such as exoplanet transmission spectroscopy, emission spectroscopy and direct imaging technology, to further study whether these habitable zone planets really have the conditions for the existence of Earth-like life."
Gu Shenghong revealed that in terms of international cooperation, in the future they will actively participate in the data analysis work of Europe's Planetary Transit and Stellar Oscillations (PLATO) mission and the ARIEL telescope project, and jointly promote the exploration of Earth-like extrasolar life with scientists around the world.
Related paper information: https://doi.org/10.1038/s41550-025-02565-z
(Original title: "About 10 times the mass of the Earth! A 'super-Earth' has been found")