Oven-Hot Exoplanet Near ‘Radius Valley’ Shows How Stars Strip Worlds Bare

An oven-hot planet twice the size of Earth and nearly as dense is giving astronomers a rare look at how stars can strip worlds nearly bare.

A Neptune-size world with Earth-like density

The newly characterized exoplanet, TOI-5734 b, circles a young orange dwarf star about 106 light-years away every 6.18 days. Despite its Neptune-like girth — about 2.1 times Earth’s radius and nine times its mass — the planet’s bulk density is almost identical to Earth’s, a surprising combination that suggests much of its original atmosphere has already been blasted into space.

In a study posted Feb. 20, an international team in the Italian-led GAPS collaboration reported that TOI-5734 b sits on the upper edge of the so‑called “radius valley,” a sparsely populated band in the size distribution of small exoplanets. That location, matched with its unusually high density, makes the world a key test case for competing theories about how close-in planets lose their atmospheres.

“TOI-5734 b has an Earth-like density and is close to having a rocky composition and an almost completely depleted primary envelope,” lead author Simone Filomeno of Italy’s National Institute for Astrophysics wrote in the paper describing the discovery. The team called the planet “a promising target” for studies of exoplanet atmospheres with current and future telescopes.

From a TESS signal to a measured mass

The planet was first spotted in 2022 by NASA’s Transiting Exoplanet Survey Satellite (TESS), which scans the sky for tiny, periodic dips in starlight as planets cross in front of their stars. The star, cataloged as TOI-5734, showed a repeating dimming every 6.18 days in three separate TESS observing sectors, flagging the system as a “TESS Object of Interest.”

To confirm the candidate and weigh it, the GAPS group turned to HARPS-N, a high-precision spectrograph on the 3.58-meter Telescopio Nazionale Galileo on La Palma in Spain’s Canary Islands. By measuring the minute wobble of the star as the planet tugged on it — only a few meters per second — the team could infer the planet’s mass and, together with the TESS-measured radius, its density.

That was harder than it sounds. TOI-5734 is a relatively young K-type star, roughly 500 million years old, about a tenth the age of the Sun. It spins once every 11 days and is magnetically active, with starspots and flares that can mimic or mask the radial-velocity signature of an orbiting planet.

To untangle those signals, the researchers used a statistical technique known as Gaussian process modeling to separate the erratic motion caused by stellar activity from the regular pull of the planet. Combining that with the TESS photometry, they arrived at a mass of about 9.1 Earths and a radius of about 2.10 Earths for TOI-5734 b. Those numbers imply a density of roughly 5.4 grams per cubic centimeter, essentially the same as Earth’s, and a surface gravity about twice Earth’s.

“It’s about two times larger and nine times more massive than Earth, with a density slightly lower than that of our home planet,” astronomer Tomasz Nowakowski wrote in a news report on the discovery that highlighted the planet’s location on the “upper edge of the so‑called ‘radius valley.’”

Why the “radius valley” matters

The radius valley, also called the radius gap, emerged from analyses of thousands of planets found by TESS and its predecessor, NASA’s Kepler mission. When astronomers plot the number of planets against their sizes, they see two peaks — one for rocky “super-Earths” between about 1 and 1.5 Earth radii, and another for lower-density “sub-Neptunes” between about 2 and 4 Earth radii. In between, near 1.5 to 2 Earth radii, there is a relative dearth of planets.

The leading explanation is that intense radiation from a star strips light, hydrogen-helium atmospheres from small, close-in worlds over hundreds of millions of years. Depending on their initial sizes and compositions, some planets keep thick gaseous envelopes and puff up into sub-Neptunes, while others lose most of that gas and shrink into bare, rocky super-Earths. A planet’s position relative to the valley offers clues about how much atmosphere it was born with and how much it has lost.

TOI-5734 b is just above that valley in size, but its density looks more like that of a planet that has already shed most of its primordial hydrogen and helium.

“Density close to Earth’s is unusual for a planet of this size” and hints that the planet has “already lost most of its light primary atmosphere,” a Ukrainian-language science outlet noted in coverage of the discovery, describing it as “a world between Earth and Neptune.”

A scorched orbit and a vanishing envelope

Orbiting at just 0.059 astronomical units — about one-fifth the distance between Mercury and the Sun — TOI-5734 b receives roughly 50 to 60 times the stellar energy Earth gets. Its calculated equilibrium temperature is around 688 kelvin (about 415°C / 779°F), hot enough to melt lead and keep any remaining atmosphere thick and turbulent.

Filomeno and colleagues modeled how quickly a planet in that position would lose gas under bombardment from high-energy X-ray and ultraviolet radiation from a young, active K dwarf. They concluded that TOI-5734 b’s light hydrogen-helium envelope is already “almost completely depleted” and may vanish entirely in the next few hundred million years, leaving a dense, hot, mostly rocky planet behind. An alternative possibility is that the planet has built up a secondary atmosphere rich in heavier molecules such as water vapor, a scenario the current data cannot rule out.

Not the “hot Neptune desert,” but a key boundary case

The new work also clarifies where the planet fits among other so-called “hot Neptunes.” Some early news accounts suggested TOI-5734 b might sit inside the “hot Neptune desert,” a region of the period–mass diagram where short-period Neptune-size planets are rare. A parallel study from a separate “K-dwarfs Survey,” however, places other planets at the edge of that desert and TOI-5734 b more cleanly on the upper boundary of the radius valley.

In practical terms, that nuance matters most to theorists fine-tuning their models of atmospheric loss. For observers, TOI-5734 b’s importance lies in its accessibility. The host star is relatively bright, with an apparent magnitude of about 9.6, and the planet transits every few days, making it a convenient target for instruments that probe exoplanet atmospheres by watching starlight filter through them.

By cataloging the planet as a “super Earth exoplanet,” NASA’s public exoplanet database has already flagged TOI-5734 b for possible future scrutiny. The GAPS team wrote that the world is a strong candidate for transmission spectroscopy with the James Webb Space Telescope and, eventually, with upcoming giant ground-based observatories, which could detect signs of any remaining light gases or heavier volatiles.

What TOI-5734 b may teach us

The discovery also underscores the role of long-running, coordinated programs in exoplanet science. The TOI-5734 b paper is labeled “LXX” — the 70th installment — in the GAPS series using HARPS-N, reflecting more than a decade of systematic follow-up of planets first spotted by space missions.

For the public, TOI-5734 b is a reminder that “Earth-like” is a slippery term. This world’s density and likely rock-and-iron interior may resemble Earth’s, but its environment does not: it is scorched by 50 Suns, crushed under double Earth’s gravity and raked by flares from a restless young star.

As observatories turn toward the system to search for traces of remaining gas, astronomers hope TOI-5734 b will help pin down how fast and how completely small planets can be stripped. The same physics that is peeling away this hot sub-Neptune’s atmosphere sets the limits on which worlds can hold onto theirs — and, by extension, on which distant planets might one day resemble our own.