Are there any objects in space that are smaller than stars but larger than planets, such as brown dwarfs?

 If you packed 70 times more mass into Jupiter, it wouldn't get any wider. It would become a brown dwarf—a "failed star" trapped in the cosmic middle ground between planets and true stars.

To understand what a brown dwarf is, it helps to look at the firm mass boundaries that define stars and planets:

• A star is an object massive enough that the crushing pressure and heat in its core ignite the sustained nuclear fusion of regular hydrogen (hydrogen-1). The absolute minimum mass required to do this is roughly 80 times the mass of Jupiter.
• A planet like Jupiter does not have enough mass to ignite any kind of nuclear fusion. It generates no internal fusion energy and simply cools down over time.

If an object forms with a mass between roughly 13 and 80 times that of Jupiter, it falls directly into this middle ground. Unlike true planets, brown dwarfs are massive enough that their core pressure ignites nuclear fusion—but only the fusion of deuterium, a heavy isotope of hydrogen. Because deuterium is scarce, a brown dwarf burns through its supply in a few million years, which is a blink of an eye in cosmic terms.

Once the fuel is gone, the fusion stops. For the rest of its long life, the brown dwarf glows faintly in the infrared spectrum from leftover heat, slowly fading and cooling until it turns pitch black.

This size paradox is one of the most unintuitive physical properties of brown dwarfs. Due to the quantum mechanics of electron degeneracy pressure, adding mass to a brown dwarf does not make it physically larger—it just makes it denser. Gravity pulls the extra mass inward so tightly that a heavy brown dwarf remains almost exactly the same diameter as Jupiter.

A size comparison showing a low-mass star, a brown dwarf, and Jupiter. Due to extreme density, a brown dwarf has roughly the same physical volume as Jupiter despite being up to 80 times more massive. Source: Wikimedia Commons.

For decades, these objects only existed in mathematical theories because they emit almost no visible light. Astronomers finally found undeniable proof in 1995 with the discovery of Gliese 229B. Spotted orbiting a red dwarf star, it glowed faintly in the infrared, and its atmosphere contained methane—a molecule that cannot survive the intense heat of a true star. At the very bottom of the brown dwarf temperature scale, astronomers have now even found Y-dwarfs, which have cooled down so much that their atmospheres match the temperatures of a warm summer day on Earth.