Astronomers Discover Sub-Neptune Exoplanet Orbiting a Sunlike Star

By peering deep into the stellar seas of our cosmic neighborhood, astronomers have discovered a glistening new world—one that challenges our ideas of planet formation and hints at a watery realm far from Earth.

In the quest to uncover the strange and beautiful worlds beyond our solar system, NASA’s Transiting Exoplanet Survey Satellite (TESS) continues to deliver cosmic surprises. The latest revelation? A shimmering sub-Neptune exoplanet orbiting a star not unlike our own sun. This alien world, dubbed TOI-3493 b, is more than three times the size of Earth and nearly nine times as massive, hinting at a mysterious, water-rich composition that could redefine how we classify exoplanets.

The discovery, spearheaded by astronomer Priyanka Chaturvedi of the Thuringia State Observatory in Tautenburg, Germany, was detailed in a study released on April 17 on the preprint server arXiv. While TOI-3493 b may not harbor life, its characteristics make it an ideal target for future exploration—especially using the powerful eyes of next-generation telescopes.

A New Jewel in TESS’s Crown

TOI-3493 b joins an ever-growing family of exoplanets detected by TESS, a space telescope launched in April 2018 with the sole mission of scouting the brightest stars near Earth in search of distant worlds. So far, TESS has flagged over 7,500 planetary candidates—known as TOIs, or TESS Objects of Interest. Of those, more than 620 have been confirmed as bona fide planets, with TOI-3493 b adding fresh luster to that number.

But what makes this new world so compelling? Unlike the cold gas giants of our outer solar system, TOI-3493 b orbits much closer to its parent star. Its “year” lasts just over eight days, and with an equilibrium temperature of 1,102 Kelvin (around 829°C or 1,524°F), it bakes in the intense starlight of a sun that sits 315 light-years away from Earth.

TOI-3493 b is part of a rare and fascinating class: sub-Neptunes—planets smaller than Neptune but bigger than Earth. These mid-sized exoplanets are among the most common in the galaxy, yet curiously absent from our own solar system. What lies beneath their hazy skies remains one of the great riddles of modern astronomy.

A Star Like Ours—Only Older and Quieter

TOI-3493, also known by its catalog name HD 119355, is a G-type main-sequence star, which places it in the same spectral category as our sun. It has a mass nearly identical to the sun and a slightly larger radius—about 1.23 times that of our solar companion. With a surface temperature of 5,844 Kelvin, it glows with familiar golden light.

Yet TOI-3493 isn’t a young star blazing with youthful fury. It’s a relatively ancient stellar elder, estimated to be around 7.3 billion years old—more than two billion years older than our sun. Astronomers measured its slow rotational period of 34 days, suggesting the star is calm, stable, and magnetically inactive—qualities that make it an ideal laboratory for studying exoplanets without interference from stellar noise.

A Planet Made of Water, Rock, and a Whisper of Hydrogen

So, what exactly is TOI-3493 b made of? Based on data gathered from both TESS and ground-based telescopes, the planet has a radius of 3.22 Earth radii and a mass of 8.97 Earth masses, which translates to a bulk density of just 1.47 grams per cubic centimeter. For comparison, Earth’s density is 5.51 g/cm³, while Neptune’s is 1.64 g/cm³. This low density rules out a purely rocky world and strongly hints at a significant fraction of water or gas in its composition.

Astronomers propose two main models for its structure. In one scenario, TOI-3493 b could have a large, silicate-and-water-based core enveloped by a thin shell of hydrogen and helium. Alternatively, it might host a denser, rocky core sheathed in a thicker hydrogen envelope. Either way, its makeup leans toward a water-rich identity—suggesting oceans not of liquid, but of supercritical water or pressurized vapor beneath a heavy, stifling atmosphere.

These models don’t just inform us about this specific planet—they help refine our understanding of how planetary systems form and evolve. Was TOI-3493 b born in its current orbit, or did it migrate inward over time? Did it once resemble a gas giant, only to lose much of its atmosphere in the glare of its host star? These are the mysteries researchers hope to unravel in the years ahead.

A Prime Candidate for Atmospheric Exploration

Beyond its intriguing structure, TOI-3493 b shines as an outstanding target for atmospheric characterization. The planet boasts a Transmission Spectroscopy Metric (TSM) score of approximately 110, making it the second-most promising known exoplanet for transmission spectroscopy—second only to HD 191939 b.

What does that mean? Transmission spectroscopy is a powerful technique in which scientists observe starlight passing through a planet’s atmosphere during a transit. By analyzing the absorbed wavelengths, astronomers can infer the atmospheric composition—detecting molecules like water vapor, carbon dioxide, methane, and possibly even biosignatures in more Earth-like planets.

TOI-3493 b’s high TSM value means that it is bright enough, big enough, and close enough to its star to enable detailed study. Instruments aboard the James Webb Space Telescope (JWST) or the upcoming Ariel mission by ESA could soon turn their attention to this steamy, strange world, offering a glimpse into what alien atmospheres are made of.

Planetary Rarity and Scientific Gold

The discovery of TOI-3493 b is not just another exoplanet added to the list—it’s a scientific goldmine. It fills a key niche in what astronomers call the “radius valley”—a puzzling gap in the distribution of exoplanet sizes where few planets are found between 1.5 and 2 Earth radii. Though TOI-3493 b is larger than this gap, it belongs to the family of planets whose origins may help explain it.

Some scientists believe that many planets start with thick hydrogen atmospheres, but only those above a certain size and mass can hold onto that gas over billions of years. The rest are stripped bare by stellar radiation, leaving behind smaller rocky cores. Studying planets like TOI-3493 b, which have likely retained some of their gas, helps test these theories in real time.

Moreover, its quiet host star allows for more accurate data collection. Stellar activity often interferes with observations, but the low magnetic variability of TOI-3493 means that future measurements of the planet’s orbit, mass, and atmosphere will be cleaner and more precise.

A Tiny World, a Vast Promise

TOI-3493 b may never be a place humans can visit. With temperatures hot enough to melt lead and pressures that could crush steel, it is far from habitable by Earthly standards. Yet in the grand tapestry of our galaxy, this sub-Neptune stands as a vital thread. It reminds us that planets come in astonishing variety—and that every new world we discover expands our sense of what’s possible.

Its study may help solve the riddle of planetary formation, open a window into alien atmospheres, and even guide our search for life by showing us how different—and how similar—other worlds can be. In this shimmering orb of gas and water, circling a sunlike star hundreds of light-years away, lies the promise of discovery—a promise that TESS and the scientists who follow its data continue to fulfill.

Reference: P. Chaturvedi et al, TOI-3493 b: A planet with a Neptune-like density transiting a bright G0-type star, arXiv (2025). DOI: 10.48550/arxiv.2504.12884