Humanity’s eternal question — “Are we alone?” — has driven astronomers to develop increasingly sophisticated methods for finding habitable exoplanets. Since the first discovery of a planet orbiting a Sun-like star in 1995, scientists have sought better ways to detect Earth-like worlds where life might exist.

An international team led by China’s Yunnan Observatories used the Transit Timing Variation (TTV) method to discover Kepler-725c — a super-Earth ten times more massive than our planet located in its star’s habitable zone. Published in Nature Astronomy, this finding demonstrates TTV’s potential where traditional techniques fall short.

For years, astronomers relied primarily on two methods: transit photometry and radial velocity. However, these approaches struggle with Earth-sized planets in year-long orbits. Transits require perfect alignment, while RV measurements need extreme precision that’s challenging for small planets.

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.

Kepler-725c is a relatively dim G9-type dwarf star with a mass of 0.95 times that of the Sun and a radius of 0.85 times that of the Sun. The star is located at a distance of about 2,473 light-years from us and is characterized by an age of about 1.6 billion years. The planet orbits its Sun-like star every 207 days, receiving 1.4 times Earth’s sunlight.

So why this TTV method matters?
• Works without perfect orbital alignment;
• Detects planets too small for RV measurements;
• Ideal for finding Earth analogs in habitable zones.

This discovery comes as major missions like ESA’s PLATO and China’s Earth 2.0 telescope prepare to launch. The proven TTV method will be crucial in their search for truly Earth-like worlds.

The collaboration included scientists from Germany’s Hamburg Observatory, Xi’an Jiaotong-Liverpool University and CAS’s Nanjing Institute.