NEW YORK, July 16: Astronomers have uncovered a previously undetected alien planet, Kepler-139f, by analyzing the orbits of known planets within the Kepler-139 star system. This hidden exoplanet is about twice the mass of Neptune and 35 times the mass of Earth, taking approximately 355 days to orbit its host star. The discovery was published on May 2, 2025, in The Astrophysical Journal Letters.

Kepler-139f had managed to remain elusive due to limitations in early detection methods used by NASA’s Kepler Space Telescope. Kepler, which identified nearly 3,000 exoplanets over its nine-year mission, primarily detected planets that passed directly between their star and Earth, causing a brief dimming of the star’s light. This allowed scientists to identify the size of these planets. However, Kepler could not detect planets that were positioned above or below the line of sight of the telescope, meaning some planets remained hidden.

However, because Kepler-139 is home to multiple planets, astronomers had the opportunity to search for worlds outside of Kepler’s original detection range. The system was known to contain three rocky super-Earths that transit their star, and later, a fourth gas giant was discovered. Astronomers observed gaps in the orbits of these planets, suggesting the presence of an additional, unseen world. By studying these orbital gaps in detail, scientists were able to infer the existence of Kepler-139f.

"The challenge isn't in finding non-transiting planets but in identifying situations where we can deduce the location of these hidden worlds," explained Caleb Lammers, a graduate student at Princeton University and co-author of the study. "It was through precise measurements of the known planets’ orbits, including radial velocity (RV) and transit timing variations (TTVs), that we could infer the existence of Kepler-139f."

The RV technique measures the gravitational pull a planet exerts on its host star, helping astronomers determine the planet’s mass. Additionally, TTVs—subtle variations in the timing of transits of known planets—can indicate the presence of other planets that don’t transit the star themselves.

Lammers and his colleague, Joshua Winn, an astrophysicist at Princeton and participating scientist on the Kepler mission, worked together to identify these gaps in the system. Using a combination of RV and TTV measurements, the pair discovered Kepler-139f nestled between the outermost super-Earth and a gas giant in the system.

Kepler-139f's discovery also provided clarity on the previously puzzling density of Kepler-139c, the outermost super-Earth in the system. The earlier density readings were incorrectly influenced by the pull of the hidden planet, but with Kepler-139f identified, the team revised the data, providing more typical density values for Kepler-139c.

The discovery raises the possibility that other unseen planets may be lurking in the Kepler-139 system. "There may still be other undiscovered planets in the system, especially given the prominent gap between planets b and c," said Lammers. "The challenge now is to find them."

While the Kepler and TESS (Transiting Exoplanet Survey Satellite) missions have been adept at detecting planets closer to their stars—where transits are more frequent and easier to observe—planets further away with wider orbits present more difficulties. Additionally, the RV method is more effective for detecting larger, more massive planets due to their stronger gravitational influence. This bias toward larger worlds has made it harder to detect smaller, more distant planets, particularly those that don’t transit their stars.

To combat these challenges, astronomers are combining multiple methods—transits, RVs, and TTVs—to identify smaller, hidden planets. "It’s likely that many planetary systems host unseen worlds, particularly in their outer regions," Lammers added.

Looking ahead, the European Space Agency’s upcoming PLATO mission, launching in 2026, will provide an even more powerful tool for detecting these hidden planets. PLATO will survey transiting planets and revisit the Kepler field, refining TTV measurements and boosting the potential for discovering more misaligned and hidden worlds.

"In the near future, TTV-based planet detection is expected to accelerate significantly with the PLATO mission," Lammers concluded.