What Are 'Forbidden' Planets and Why They Exist

A Planet That Shouldn't Be There

Imagine a pea orbiting a lemon. That is roughly the size ratio between the gas giant TOI-5205 b and its host star, a red dwarf barely four times wider than Jupiter. By every mainstream model of how planets form, a world this massive should not exist around a star this small. Astronomers call objects like it "forbidden" planets—gas giants found in places theory says they cannot grow.

The discovery, first flagged by NASA's Transiting Exoplanet Survey Satellite (TESS) and later confirmed by ground-based telescopes, is not an isolated curiosity. A growing catalogue of similar mismatches is forcing planetary scientists to revisit the fundamental physics of how worlds are born.

How Planets Normally Form

The dominant explanation for gas giant formation is the core accretion model. In the swirling disk of gas and dust around a newborn star, tiny grains collide and stick together, gradually building a rocky core. Once that core reaches roughly 10 to 15 Earth masses, its gravity becomes strong enough to sweep up vast quantities of surrounding hydrogen and helium, ballooning into a gas giant like Jupiter or Saturn.

The process is slow—estimates range from a few million to tens of millions of years—and it demands a disk rich in solid material. That is precisely the problem with red dwarfs. These cool, dim stars, which make up about 70 percent of all stars in the Milky Way, have proportionally less massive disks. Less raw material means less chance of building a core heavy enough to trigger runaway gas capture before the disk dissipates.

Why Red Dwarfs Break the Rules

Surveys suggest that close-in gas giants orbit only about one in 40 red dwarfs, compared with roughly one in ten Sun-like stars. The rarity fits the core accretion prediction—but the exceptions are spectacular. TOI-5205 b blocks a full seven percent of its star's light during each transit, an enormous signal that leaves no doubt about its size.

Observations by the James Webb Space Telescope (JWST) have deepened the mystery. A 2025 study published in The Astronomical Journal found that TOI-5205 b's atmosphere contains fewer heavy elements than its own host star—the opposite of what core accretion predicts. Interior models suggest the planet's bulk is roughly 100 times more metal-rich than its thin outer envelope, implying the interior and atmosphere are not well mixed.

The Alternative: Disk Instability

If a core cannot grow fast enough, perhaps the planet never needed one. The disk instability model proposes that a sufficiently massive and cool protoplanetary disk can fragment directly into self-gravitating clumps. These clumps collapse under their own weight in as little as a few hundred years—thousands of times faster than core accretion.

Disk instability elegantly explains how a gas giant could appear around a low-mass star before the disk vanishes. However, the model has its own difficulties: it requires specific disk temperatures and masses, and simulations show that many clumps are sheared apart before they can contract into stable planets.

A third possibility involves longer-lived disks around red dwarfs. Gas-rich disks have been detected around low-mass stars as old as 45 million years, far longer than the typical five-to-ten-million-year lifespan seen around Sun-like stars. A longer window could give core accretion the extra time it needs.

Why It Matters

Red dwarfs are the most common stars in the galaxy, and many sit in the crosshairs of exoplanet surveys searching for habitable worlds. Understanding what kinds of planets can form around them—and how—directly shapes estimates of where life might arise.

Forbidden planets also serve as natural laboratories. Because they block such a large fraction of their star's light, instruments like JWST can study their atmospheres in unusual detail, testing formation models with real chemical fingerprints rather than computer simulations alone.

As the catalogue of these misfit worlds grows, one thing is clear: the universe builds planets in ways astronomers have not yet fully imagined.