Astronomers have found a concealed super-Earth in the habitable zone of a distant Sun-like star, using a groundbreaking detection technique. The discovery could reshape the hunt for life beyond our planet. (Artist’s concept). Credit: SciTechDaily.com
A new detection method reveals a potentially habitable super-Earth. The breakthrough boosts the search for life-supporting planets.
“Are we alone?” This enduring question has fascinated humanity for generations. The discovery in 1995 of the first exoplanet orbiting a star similar to our Sun marked a pivotal moment in addressing that mystery.
Since then, exoplanet research has become a central focus of modern astronomy, offering critical insights into how planets form, evolve, and possibly support life. A major objective in this field is finding Earth-like life, which hinges on locating planets with Earth-like conditions in the habitable zones of Sun-like stars.
Artist’s view of the Kepler-725 system. The small planet in the lower right is the newly discovered super-Earth in the habitable zone. Credit: Shenghong Gu
Breakthrough discovery using TTV
In their search for Earth-like worlds, an international team led by the Yunnan Observatories of the Chinese Academy of Sciences (CAS), in collaboration with other institutions, made a significant breakthrough using a method called Transit Timing Variation (TTV).
For the first time, TTV enabled the detection of a super-Earth named Kepler-725c, which is about 10 times the mass of Earth and orbits within the habitable zone of a Sun-like star, Kepler-725. The findings were published in Nature Astronomy.
Traditionally, astronomers have used the transit method and radial velocity (RV) measurements to detect low-mass planets (10 Earth masses or less) within the habitable zones of Sun-like stars. However, these smaller planets usually follow long orbits and produce weak RV signals, making them difficult to detect. The RV technique, in particular, demands extremely precise measurements, which limits its practicality for identifying such faint, distant planets.
Light curves and TTV pattern of the transiting planet Kepler-725b. The TTV inversion reveals the presence of an additional super-Earth, Kepler-725c, in the system. Credit: Shenghong Gu
The transit method faces its own challenges due to geometric limitations. It only works when a planet’s orbit aligns perfectly with the observer’s line of sight, a condition that is uncommon for planets with long orbital periods. Even when these alignments occur, the dim and brief light changes they cause are often too subtle to be confidently identified, leading to a high chance of missing potential discoveries.
Kepler-725c’s orbit and habitability
Kepler-725c—the newly discovered non-transiting planet—orbits a G9V host star. With an orbital period of 207.5 days and a semi-major axis of 0.674 AU, it receives roughly 1.4 times the solar radiation that Earth does. During part of its orbit, the planet lies within the host star’s habitable zone, making it a potential candidate for habitability.
By analyzing the TTV signals of Kepler-725b, a gas giant planet with a 39.64-day orbit in the same system, the team successfully inferred the mass and orbital parameters of the hidden planet Kepler-725c, demonstrating the potential of the TTV technique to detect low-mass planets in habitable zones of Sun-like stars.
Unlike the transit and RV methods, the TTV technique does not require the planet’s orbit to be edge-on or rely on high-precision RV measurements of the host star. This makes the TTV technique particularly well-suited for detecting small, long-period, non-transiting habitable planets that are otherwise difficult to discover using these other two methods. Thus, the TTV method fills a critical gap among current detection techniques, providing a promising alternative for discovering “Earth 2.0.”
Based on the results of this study, once the European PLATO mission and Chinese ET (“Earth 2.0”) mission are operational, the TTV method is expected to greatly enhance the ability to detect a second Earth.
Reference: “A temperate 10-Earth-mass exoplanet around the Sun-like star Kepler-725” by L. Sun, S. Gu, X. Wang, J. H. M. M. Schmitt, P. Ioannidis, M. B. N. Kouwenhoven, J. Dou and G. Zhao, 3 June 2025, Nature Astronomy.
DOI: 10.1038/s41550-025-02565-z
This work is supported by the National Natural Science Foundation of China and the Yunnan Fundamental Research Project.