Astronomers have found another super-Earth. It's about 10 times more massive than Earth, and orbits in the habitable zone of a Sun-like star 2475 light-years away. These massive Earth-like planets hold key information about how planets form and evolve.
The planet is named Kepler-725c, and despite being 10 times more massive than Earth, it's still considered a low-mass exoplanet. Super-Earths like this are intriguing objects because our Solar System has none, even though they're the most common type of exoplanet found, with more than 1500 confirmed so far. They're larger than Earth, smaller than Neptune, and are typically 1.25 to 2 Earth radii.
The discovery is in a new research article in Nature Astronomy titled "A temperate 10-Earth-mass exoplanet around the Sun-like star Kepler-725." The lead author is L. Sun from the Chinese Academy of Sciences.
What's remarkable about this discovery is that astronomers used the Transit Timing Variation (TTV) method to detect it. Typically, low-mass exoplanets like this are found using the transit or radial velocity methods.
The transit method can only be used when an exoplanet blocks starlight from our point of view, so the solar systems need to be edge-on from our perspective. Since the method relies on blocked light, larger planets are preferentially detected since they block more light. Planets on close orbits are also preferentially detected because they complete more transits in less observation time.
Radial velocity, or Doppler spectroscopy, is also biased toward more massive planets closer to the star. It works by sensing the shifting wavelengths of light from a star as the planet induces a slight wobble in the star. Since smaller planets induce smaller wobbles than massive planets, massive planets are preferentially detected.
The TTV method uses the presence of a known exoplanet to detect an additional planet. In this case, astronomers already knew of one planet in the system named Kepler-725b. It's a gas giant exoplanet about 1/4 the mass of Jupiter. Astronomers watched it orbiting its star and looked for variations in the timing of its orbit. Any changes signal the presence of an additional planet, in this case, its super-Earth sibling Kepler-725c. Since the exoplanet Kepler-725b is much less massive than the star, it's affected by its sibling more than the star is. That makes the effect easier to observe.
Each method has strengths and weaknesses, but the transit and radial velocity methods are biased towards larger planets with short orbital periods of about 100 days or less. But Kepler-725c has an orbital period of 207.5 days.
This figure from the research shows some of the results. Black solid dots represent the observed TTVs for the known exoplanet Kepler-725b. The line with bright green diamonds represents the best orbital period for Kepler-725c of 207.5 days. Image Credit: Sun et al. 2025. Nature Astronomy.
TTV reveals more than just the presence of an additional planet. The astronomers also extracted important information as it tugged on its sibling. "Through analysis of the transit timing variations of the relatively short-period (39.64 days) warm Jupiter Kepler-725 b, we find that Kepler-725 c has a period of 207.5 days and travels in an eccentric orbit (with an eccentricity of 0.44 ± 0.02 and an orbital semi-major axis of 0.674 ± 0.002 au), receiving a time-averaged insolation of 1.4 times the Earth's value," the researchers explain.
The planet's eccentric orbit means it spends only part of its time in the habitable zone, where liquid water may persist on its surface. With only this data to go by, its habitability is an open question. Depending on its atmosphere and other factors, it's possible that some type of life could survive, but that's entirely speculative.
"This discovery demonstrates that the transit timing variation method enables the detection and accurate mass measurement of a super-Earth/mini-Neptune within a solar-like star's habitable zone," the authors write in their article.
The fact that TTV successfully found this planet is a positive development. By combining TTV observations with other methods, astronomers can get a much clearer picture of a distant solar system. TTV observations are particularly strong at determining solar system architectures, while radial velocity excels at revealing planetary masses, and the transit method determines planet sizes.
Exoplanet science is shifting from an emphasis on detection to a focus on characterizing exoplanets and their solar systems. The TTV method shows promise for filling in the gaps in solar systems already known to host exoplanets. When the method is combined with an upcoming mission like the ESA's PLATO spacecraft, our search for super-Earths and Earth-size worlds will take a big step forward.
Citation: Sun, L., Gu, S., Wang, X. et al. A temperate 10-Earth-mass exoplanet around the Sun-like star Kepler-725. Nat Astron (2025). https://doi.org/10.1038/s41550-025-02565-z
