For decades, scientists have scanned the cosmos in search of distant planets and possible signs of extraterrestrial life. This relentless exploration has yielded many fascinating discoveries and driven the development of increasingly advanced instruments. However, planets that closely resemble Earth—especially those with low mass—often manage to evade detection.
Many of these elusive planets remain undetected due to the limitations of conventional observation methods. Their orbital alignments may not suit our line of sight, or their faint signals might fall below the threshold of standard detection techniques. These shortcomings have long posed a challenge to astronomers trying to discover Earth-like planets in faraway solar systems.
In a significant step forward, Leilei Sun, the lead author from Yunnan Observatories of the Chinese Academy of Sciences, along with a team of international collaborators, recently confirmed the existence of a super-Earth dubbed Kepler-725c. This discovery was made possible by a unique strategy that sidesteps the limitations of the widely used transit and radial velocity methods.
There are several established ways to detect planets outside our solar system, also known as exoplanets. One of the most popular methods is the transit technique, which involves observing slight dips in a star’s brightness caused by a planet passing in front of it. These dips signal the presence of a planet and provide information about its size and orbit.
This technique is particularly effective for identifying large exoplanets with short orbital periods. These planets pass across their host stars frequently, making them relatively easy to detect. Kepler-725c, for instance, belongs to this category of big, short-period planets. However, smaller planets with longer orbital cycles are more difficult to detect with the transit method. Their rare alignments with Earth’s line of sight make them much harder to observe.
That’s why Kepler-725c’s detection has drawn attention. Researchers are especially interested in planets with up to 10 times the mass of Earth. These so-called super-Earths are thought to form differently from much larger gas giants and may possess characteristics similar to our own planet. A mass close to Earth’s increases the likelihood of interesting features such as rocky terrain or the ability to retain water—both critical components when evaluating a planet’s potential to support life.
In order to find Kepler-725c, scientists employed the transit timing variation method, or TTV. This technique monitors how a planet’s gravity influences the orbit of a neighboring planet, causing slight shifts in its expected transit times. According to Sun, “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 team studied changes in the transit times of Kepler-725b, a gas giant similar to Jupiter, to identify Kepler-725c in the same planetary system. The gravitational interplay between the two planets provided the telltale evidence of Kepler-725c’s existence.
One of the key advantages of TTV is that it doesn’t require the planet being studied to pass directly in front of its star from our point of view. Nor does it rely on detecting minute shifts in the star’s velocity caused by the gravitational tug of an orbiting planet. As such, TTV opens a door to finding planets that would otherwise be invisible.
This technique is particularly effective in systems where only one planet is seen transiting, but its movement suggests the presence of another gravitational body. These indirect signs, similar to cosmic breadcrumbs, lead researchers to unseen planetary companions. In the case of Kepler-725c, scientists were able to determine its orbit and mass even without visually detecting its transit.
Kepler-725c is located roughly 2,472 light-years from Earth. It orbits a G9V-type star and completes one full revolution in about 207.5 days. Its path occasionally takes it through the habitable zone—the region around a star where conditions might allow liquid water to exist. It receives about 1.4 times the solar radiation Earth gets from the Sun at a distance of 1 astronomical unit.
With an orbital distance of approximately 0.674 AU, Kepler-725c may experience moderate surface temperatures. However, many additional factors—such as atmospheric composition, planetary rotation, and magnetic fields—play a role in determining whether the planet could truly be habitable. Scientists aim to explore how heat, star behavior, and atmospheric makeup might affect Kepler-725c as they continue their analysis.
The timing of this discovery is significant. Space agencies around the world are preparing for missions that will focus on detecting smaller planets around Sun-like stars. Europe’s PLATO mission, among others, is expected to generate data that complements TTV-based methods. These upcoming missions could reveal additional Earth-like planets in similar orbital zones.
This moment marks a crucial opportunity for astronomers to refine their understanding of what conditions are necessary for life. By determining a planet’s mass and orbit with precision, TTV allows researchers to assess its characteristics without the limitations of traditional observation strategies.
The discovery of Kepler-725c demonstrates the practical value of the TTV method in identifying planets that do not visibly transit their stars. These hard-to-see worlds might still meet critical criteria for habitability, and TTV offers a powerful approach to locating them.
Future space missions could work hand-in-hand with this technique to uncover more low-mass, long-orbiting planets that older detection methods have missed. Such findings have the potential to greatly sharpen our focus as we search for planets that might support life.
Still, even with better detection tools and refined techniques, verifying whether a planet is truly habitable remains a complex and slow-moving process. For planets like Kepler-725c, more data—especially direct imaging or atmospheric readings—are needed before scientists can determine if life might exist there. So far, researchers mostly have indirect clues such as mass, orbit, and radiation levels, which are informative but not definitive.
Vital elements like liquid water, oxygen, or a stable surface are necessary for life as we know it. These details are still beyond our reach for many newly discovered planets, including Kepler-725c.
The research team behind this discovery includes scientists from several institutions: Yunnan Observatories, Hamburg Observatory, Xi’an Jiaotong-Liverpool University, and the Nanjing Institute of Astronomical Optics and Technology. Their international collaboration reflects a growing global interest in using advanced techniques to uncover distant planets and better understand their environments.
By combining gravitational measurements with long-term orbital data, these researchers have illuminated yet another small corner of our universe—bringing us one step closer to answering the age-old question: are we alone?