A Puffy Planet: Inflated and Super-Light Exoplanet Discovered

Original: "Discovery of an Inflated Hot Neptune and Its Formation from Jovian Mass Loss"

arXiv:2607.01315v1 · 2026-07-01 · CC BY · ⏱ 2 min · Exoplanets

Astronomers have found a planet that resembles a giant balloon—it's light but enormous, and this helps explain how Neptunes are born around alien suns.

Abstract

Planet TOI-2195 A b is a bloated hot Neptune that was likely born as a large gas giant but lost almost all its mass by passing too close to its star. Its orbit is steeply tilted, like a swing pushed by a distant stellar companion. This explains where some mysterious hot planets come from. Could our own Solar System hide a similar story?

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Astronomers have reported the discovery of an incredible exoplanet—a planet orbiting another star. The new planet, TOI-2195 A b, turns out to be a real cosmic balloon. Imagine: it's almost the size of Jupiter but weighs only slightly more than Neptune. Its density is so low that it could float on water.

The planet is so close to its star that it completes an orbit in just four days—a year there lasts less than an Earth week.

So how did such a 'fluffy' planet come to be? Using the transit method—when a planet briefly blocks some of its star's light—scientists measured its size. This method was first widely used by William Borucki in the Kepler space mission. To find the mass, they turned to spectroscopy—analyzing the star's light broken into colors, like a rainbow. Studying the composition of stars this way was pioneered by Cecilia Payne-Gaposchkin. It turned out the planet is mainly made of hydrogen and helium—the lightest gases—but it's still far too puffed up for its mass. Models show it probably started out as a true Jupiter, but during its migration toward the star it lost almost all its gas—like a burst balloon that's let out nearly all its air yet still holds its shape.

It's like inflating a large rubber balloon (a Jupiter), then releasing 90% of the air—it would shrink but still be bigger than a typical inflatable ball of that weight (a Neptune). Because it was hot inside, it cools slowly and stays swollen.

The discovery helps solve a long-standing puzzle: why are there so few hot planets with Neptune-like masses? It seems many hot Neptunes are 'slenderized' Jupiters that survived partial destruction. In the future, the new James Webb Space Telescope will look for heavier molecules like water in their atmospheres to test these ideas. Meanwhile, scientists are contemplating how internal heat—entropy (a measure of disorder)—allows such planets to avoid shrinking for a long time.

🎯 If you could place this planet in a gigantic ocean, it wouldn't sink—its density is five times less than water's. A true cosmic float!

r_t = R_p \left(\frac{M_\star + m_p}{m_p}\right)^{1/3}

The distance from the planet to the star at which tidal forces tear the planet apart.

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A Puffy Planet: Inflated and Super-Light Exoplanet Discovered