# Rare super-Jupiter spotted on a six-month orbit

> A study in Monthly Notices has identified NGTS-38 b, a rare giant world that takes about 180 days to circle its star. Led by scientists at Queen's University Belfast, the international team found a planet about 8% wider than Jupiter and nearly...

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Byline: Queen’s University Belfast
Published: 2026-07-18T11:15:02+00:00
Categories: News, Space

![Artist-style view of a gas giant exoplanet and moon in space](https://www.argo.net/wp-content/uploads/2026/07/gas_giant_exoplanet_space.jpg)

A study in [Monthly Notices](https://academic.oup.com/mnras/article/549/4/stag1061/8703206) has identified NGTS-38 b, a rare giant world that takes about 180 days to circle its star. Led by scientists at **Queen's University Belfast**, the international team found a planet about 8% wider than Jupiter and nearly five times as massive.

The planet, formally cataloged as TIC-65910228 b and also known as **NGTS-38 b**, belongs to a small group of transiting warm giants with long orbital periods. These worlds are difficult to catch because they pass in front of their stars only occasionally. That makes this discovery a valuable new target for studying giant planets at cooler temperatures than many well-known close-in exoplanets.

Toby Rodel, a Ph.D. student in the School of Mathematics and Physics at Queen's, led the discovery under the supervision of Professor Christopher Watson. "This has been an incredible discovery," Rodel said. The result came from years of patient monitoring, beginning with a single dimming event recorded by NASA's planet-hunting satellite TESS.

## A giant planet hiding in slow motion

**NGTS-38 b** is a **super-Jupiter**, a term used for giant planets that exceed Jupiter in mass. The study reports a radius of 1.081 Jupiter radii and a mass of 4.77 Jupiter masses. In plain terms, the planet is only slightly larger than Jupiter across, yet it is far heavier.

That combination points to a dense gas giant with strong gravity. Since it is far enough from its star to avoid the extreme heating seen on many hot Jupiters, NGTS-38 b gives astronomers a chance to study a giant world in a different thermal environment. The paper estimates an equilibrium temperature of about 457 Kelvin, which is roughly 184 degrees Celsius.

Its year lasts 180.52797 days. That makes it one of the longer-period planets found through the transit method, according to the research team. "Finding one much farther out at 180 days is a big deal!" Rodel said.

The planet orbits a bright F6V to F7V type host star. This star is larger and hotter than our Sun and it has a high metal content compared with the Sun. In astronomy, metals mean elements heavier than hydrogen and helium. Such ingredients matter because they help build the cores and atmospheres of planets.

## The Christmas Day signal

The discovery began with a single dip in starlight on Christmas Day 2020. NASA's **TESS**, short for Transiting Exoplanet Survey Satellite, spotted the event while scanning the sky for small changes in stellar brightness. A dip like this can occur when a planet crosses the face of its star from our point of view.

This technique is called the **transit** method. It has transformed exoplanet science because it can reveal a planet's size and orbital timing. When a planet repeatedly blocks a tiny fraction of its star's light, astronomers can measure how wide the planet is and how often it returns.

Long-orbit planets create a tougher puzzle. A world that circles its star every few days gives observers many chances to see another transit. A planet with a six-month year gives them only a few chances across several years. If weather, daylight, or telescope scheduling gets in the way, an important event can be missed.

The first TESS signal gave the team a promising clue, yet one transit alone could not establish the orbit. Astronomers needed to catch the planet again and measure the star's motion. That turned the discovery into a long campaign across multiple observatories and research groups.

## More than 200 nights of follow-up

Researchers then turned to the **Next Generation Transit Survey**, known as NGTS, in Chile. The team monitored the star for more than 200 nights. Their persistence paid off when NGTS caught the final moments of a second transit.

That second transit helped lock down the timing. The study reports that a photometric monitoring campaign of 228 nights detected a transit egress. Egress is the moment when the planet finishes moving across the star's disk and the star's light returns to normal.

The team also used **radial velocity** observations. This method measures tiny shifts in starlight caused by a planet's gravitational tug. Even a massive planet does not drag its star very far, but modern instruments can detect the subtle back-and-forth motion.

Spectroscopic monitoring with **CORALIE and HARPS** helped confirm the planet's mass and orbit. By combining the light-dimming data with the star's wobble, the team could determine both the planet's size and its heavy mass. That combined approach is especially powerful because it separates planetary impostors from real giant worlds.

"This discovery was the culmination of years of detective work," Professor Christopher Watson said. The phrase fits the observing strategy. Astronomers had to connect a rare first clue, a hard-won second event and precision measurements of stellar motion.

## An oval orbit around a hotter star

The planet follows a **180.52797-day orbit** that is moderately eccentric. The study gives an eccentricity of 0.3086. A perfectly circular orbit has an eccentricity of zero, while higher values describe more elongated paths.

That shape means NGTS-38 b's distance from its star changes noticeably during the planet's year. At closest approach, it lies only slightly farther from its star than Mercury does from the Sun. At its farthest point, it reaches almost as far out as Earth's orbit around the Sun.

The host star changes the thermal story. Since the star is larger and hotter than the Sun, NGTS-38 b receives more stellar energy than a planet would at the same distance around our own star. Even near the outer part of its orbit, the planet remains much warmer than Earth.

This **eccentric orbit** may preserve clues about how the system formed. Giant planets can be pushed into elongated paths through gravitational interactions with other planets, migration through a young disk, or encounters earlier in the system's history. The current study identifies the orbit clearly, while the deeper formation story will require more work.

Another team, led by Felipe Rojas and Dr. Rafael Brahm at Universidad Adolfo Ibáñez in Santiago, Chile, independently identified the same system and released a preprint on arXiv. "This planet stands out among the transiting exoplanets known to date," Rojas said. Its mass, half-year orbit and mild eccentricity make it a strong case for studying the origin of giant planets.

## A possible place to hunt for moons or rings

NGTS-38 b's mass gives it a strong gravitational reach. Because it is also farther from its star than many intensely irradiated hot Jupiters, researchers note that it could be an interesting system for searches involving **moons or rings**. No such structures have been confirmed around an exoplanet so far.

Moons and rings would be extremely hard to detect. They would likely produce very small changes in the shape or timing of a transit. Still, a massive planet on a wider orbit gives astronomers a better kind of laboratory than a giant planet skimming the surface of its star every few days.

The system's brightness also helps. A bright host star allows telescopes to collect cleaner data. That makes NGTS-38 b a promising candidate for future monitoring, especially if astronomers can observe full transits with high precision.

The discovery also shows how exoplanet surveys are pushing beyond the easiest finds. Many early transit discoveries were planets on very short orbits because they crossed their stars often. NGTS-38 b shows that patient follow-up can extend the same method toward cooler, slower worlds.

For astronomers searching for planets more like those in our own solar system, that shift matters. Long-period transiting planets are rare, yet they carry information about planetary architecture across wider orbits. NGTS-38 b is a giant outlier with a six-month year and its slow passage across a bright star gives researchers a new way to probe how massive planets form and survive.