A Saturn-Sized Planet Just Revealed Itself Around Two Dim Stars 22,800 Light-Years Away

Astronomers have found a Saturn-mass world circling two red dwarf stars, a configuration so rare that conventional planet-hunting methods would have missed it entirely. The discovery, announced this week in the Publications of the Astronomical Society of the Pacific, relied on gravitational microlensing — a technique responsible for fewer than 5% of all known exoplanets despite its remarkable ability to spot worlds invisible to other approaches.

The planet, cataloged as KMT-2016BLG-1337L, sits roughly 7,000 parsecs from Earth. When researchers modeled its characteristics from the light-curve data, the results split into two possibilities: one scenario places the planet at about 0.3 Jupiter masses orbiting at 4 astronomical units from its host star, while an alternative interpretation suggests 7 Jupiter masses at just 1.5 AU.

Both models agree on the host stars themselves — a binary pair of M-dwarfs weighing in at 0.54 and 0.40 solar masses, separated by roughly 3.5 AU.

Two Possible Configurations

Researchers identified two equally plausible scenarios for this distant world:

Scenario A: A planet of approximately 0.3 Jupiter masses, orbiting its host star at a distance of 4 astronomical units

Scenario B: A more massive planet of roughly 7 Jupiter masses, situated much closer at 1.5 astronomical units

Microlensing works by using a foreground star's gravity as a magnifying glass, warping spacetime and brightening the light from a star behind it. When a planet drifts through this gravitational corridor, it leaves a detectable signature.

This requires a precise alignment of three bodies — a rarity that makes microlensing powerful for finding planets in otherwise inaccessible regions, but also explains why the method has uncovered only about 250 worlds out of the 6,100-plus exoplanets confirmed to date.

The Microlensing Advantage

Unlike transit surveys that watch for starlight dips as planets pass in front of their stars, microlensing can detect planets in regions where other techniques simply cannot reach. The method exploits Einstein's prediction that mass curves spacetime, causing background starlight to bend and brighten when a foreground object passes through the line of sight.

A Comparison With History

The new discovery echoes an earlier milestone: OGLE-2007-BLG-349L, the first confirmed exoplanet in a binary star system, also Saturn-sized and identified through microlensing, published in The Astronomical Journal in 2016.

The critical difference lies in orbital architecture. OGLE-2007-BLG-349L circles both of its host stars simultaneously, while KMT-2016BLG-1337L orbits just one. The latter case demonstrates that planets can form and persist around a single star within a binary system, undisturbed by their stellar neighbor's gravitational influence.

Microlensing underscores the capability to reveal planets in dynamically complex stellar environments, including systems that are inaccessible to conventional detection techniques.

For a method that often plays second fiddle to the transit technique, this is a reminder that the universe contains planetary configurations scientists haven't yet learned to systematically hunt.

Based on: Discovery of a Saturn-Mass Planet in a Binary Stellar System; Korean Microlensing Team; Publications of the Astronomical Society of the Pacific.