Chinese and German scientists have utilized the Transit Timing Variation (TTV) technique to locate an exoplanet super-Earth for the first time, validating a new tool that significantly broadens the possibilities for exoplanet detection.
Since the first discovery of an exoplanet orbiting a sun-like star in 1995, astronomers have been seeking Earth-like planets residing in their star’s habitable zone. The latest find, Kepler-725c, is located within its star’s habitable zone but is 10 times the mass of Earth.
Standard Techniques in the Hunt for Exoplanets
Over the decades since that first exoplanet discovery, the transit and radial velocity methods have been the primary tools for exoplanet hunters seeking low-mass planets sitting within their star’s habitable zone. On this scale, “low mass” means anything up to about 10 times the mass of Earth. The difficulty in employing these techniques is that such low-mass objects usually have long orbital periods, and the radial velocity signals they emit tend to be weak. Exacerbating the weak signal issue is the radio velocity method’s need for extremely precise data, making detections highly challenging.
The other common method, using transit, also suffers severe limitations as the exoplanet’s orbital plane must be in perfect alignment with the astronomer’s line of sight. Few long-period orbits are so optimally situated. Even when they are, their shallowness and brief durations make them hard to detect, leaving many planets overlooked with the transit method.
Transit Timing Variation
Astronomers at the Yunnan Observatories of the Chinese Academy of Sciences (CAS), led the new study, utilizing the TTV method. While transit requires the perfect view and radio velocity relies on exacting measurement precision, TTV is not so finicky. Challenging to identify, non-transiting planets with long orbital periods and small masses are much easier to detect with TTV, potentially filling in a major gap in exoplanet research.
Observing the Hidden Planet
Intriguingly, Kepler-725c remains hidden from direct observations, blocked by its orbital neighbor, Kepler-725b. With quite different characteristics from the super-Earth, Kepler-725b is a gas giant orbiting the host star every 39.64 days. Analysis of the gas giant’s TTV signals allowed the team to identify the super-Earth’s characteristics even though they could not directly observe it.
The Astronomers discovered that Kepler-725c is a non-transiting planet, orbiting partially through the habitable zone of a G9V host star over the course of 207.5 days. The super-Earth is bombarded with about 1.4 times as much solar radiation as Earth receives and is possibly habitable.
“This newly discovered non-transiting planet and its host star are located at a place about 2,472 light-years from the solar system,” said co-author Gu Shenghong, team leader from the Yunnan Observatories of the Chinese Academy of Sciences.
“More crucially, this planet resides within the habitable zone of its host star, the region around a star where the temperature is suitable for liquid water to exist,” Gu added.
Continuing with Exoplanet Research
With this study proving the functionality of TTV for exoplanet identification, astronomers have a major new tool for their work. The European PLATO mission and Chinese ET mission are both expected to provide data of great use to scientists employing TTV.
“It demonstrates the potential of the TTV technique to detect low-mass planets in habitable zones of Sun-like stars,” said first author Sun Leilei, also of Yunnan Observatories.
The paper “A Temperate 10-Earth-Mass Exoplanet Around the Sun-like Star Kepler-725” appeared on June 03, 2025, in Nature Astronomy.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.
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