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"Are we alone?" This ancient question has occupied humanity's mind for a long time. In 1995, the discovery of the first exoplanet orbiting a sun-like star opened the door to exploring this profound mystery.

The study of exoplanets has now become one of the most critical scientific topics of the 21st century, with profound implications for understanding planetary formation, evolution, and the origin of life. Searching for Earth-like life remains the ultimate goal of planetary science, and identifying Earth-like planets in the habitable zones of sun-like stars is a key step.

In pursuing this exploration, an international research team led by the Yunnan Observatories of the Chinese Academy of Sciences (CAS), along with collaborators, has achieved a breakthrough by using the Transit Timing Variation (TTV) technique for the first time to discover a super-Earth.

The exoplanet, Kepler-725c, has 10 times the mass of Earth and is located in the habitable zone of the sun-like star Kepler-725. The discovery is in Nature Astronomy.

For decades, astronomers have relied on techniques like the transit method and radial velocity (RV) observations to identify low-mass planets (≤10 Earth masses) within the habitable zones of sun-like stars. However, these low-mass celestial bodies typically exhibit long orbital periods and generate faint RV signals. Compounding this challenge, the RV method's requirement for extremely high-precision measurements limits its effectiveness in detecting small, long-period bodies.

In contrast, the transit approach is marked by geometric constraints: It requires a planet's orbital plane to align precisely with our line of sight—a rare occurrence for long-period systems. Even when such transits occur, the resulting photometric signals are often too shallow and short-lived to be reliably detected, increasing the risk of observational oversight.

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.