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The answer depends on whether you mean is any planet bigger than any star (Case 1), or whether the planet and star have to be in the same system and have been discovered/measured (Case 2), rather than just that they could exist in principle.

The answer to Case 1 is yes - planets can be demonstrably bigger than stars. The answer to Case 2 is that whilst such systems may exist in principle, they are probably rare and there are not yet (in late 2024) any reported examples.

An interesting suggestion is that a young exoplanet might offer the best chance of being bigger than its host star. This is because the contraction timescale of a giant planet is longer than the pre main sequence contraction timescale of its star. The curves in the plot above for 1 Gyr and 10 Gyr show this effect, but it is even more extreme for ages $0.1$ Gyr. Thus the best chance of finding planets bigger than their host stars is to look at young systems in star forming regions. Some of these may already have been found using direct imaging, though in my opinion these quite high-mass "exoplanets" ($>5$ Jupiter masses) orbiting at very large distances ($>100$ au) are more like binary brown dwarfs.

The answer depends on whether you mean is any planet bigger than any star (Case 1), or whether the planet and star have to be in the same system and have been discovered/measured (Case 2), rather than just that they could exist in principle. There is also now a Case 3 which would be planets orbiting compact objects like neutron stars and white dwarfs.

The answer to Case 1 is yes - planets can be demonstrably bigger than stars. The answer to Case 2 is that whilst such systems may exist in principle, they are probably rare and there are not yet (in late 2024) any reported examples. The answer to Case 3 is yes; there are now several planets known orbiting both neutron stars and white dwarfs that are almost certainly bigger than the compact star.

An interesting suggestion is that a young exoplanet might offer the best chance of being bigger than its host star. This is because the contraction timescale of a giant planet is longer than the pre main sequence contraction timescale of its star. The curves in the plot above for 1 Gyr and 10 Gyr show this effect, but it is even more extreme for ages $0.1$ Gyr. Thus the best chance of finding planets bigger than their host stars is to look at young systems in star forming regions. Some of these may already have been found using direct imaging, though in my opinion these quite high-mass "exoplanets" ($>5$ Jupiter masses) orbiting at very large distances ($>100$ au) are more like binary brown dwarfs.

Case 3

There are now examples of planets that orbits compact stars (neutron stars and brown dwarfs). In fact some of the first exoplanets discovered were around pulsars. Even the least massive of these "pulsar planets" is likely to be bigger than the $\sim$10 km radius of the neutron star it orbits.

There are also now examples of giant planets found around white dwarfs. Since white dwarfs are around the same size of the Earth, a giant planet would certainly be larger. The difficulty is in constraining the mass of the potential planet to be less than that of a brown dwarf (roughly 14 Jupiter masses). An example is WD 1856+534, which eclipses the brown dwarf and very likely has a mass less than 14 times that of Jupiter and is roughly the same size as Jupiter.

JWST has also directly imaged two planetary candidates around two white dwarfs (Mullally et al. 2024). These are likely to have mass in the range 1-7 Jupiter masses and must be far bigger than the white dwarfs, or they wouldn't be visible at all.

The answer depends on whether you mean is any planet bigger than any star, or whether the planet and star have to be in the same system and have been discovered/measured, rather than just that they could exist in principle.

If one demands that the exoplanet and star are part of the same system, then although they could exist in principle (as per the discussion above), there aren't any examples (yet). The curves in the plot above are not dependent on the type of star a planet orbits. Therefore, in principle, it might be possible for a $>1M_J$ planet to be found orbiting a (only just) smaller $<0.1 M_\odot$ star, even if it receives negligible insolation.

In practice, giant exoplanets are rare around low mass stars, so it could be some time before an example is found. However, a close candidate might be GJ3512b which is an exoplanet with $M\sin i = 0.46 M_{\rm Jup}$ (i.e. this is a minimum mass, since the orbital inclination $i<90^{\circ}$) that orbits an M5.5V star quite similar to Proxima Cen (Morales et al. 2019). The star has an estimated radius of $(0.139 \pm 0.005) R_\odot$ and the age is thought to be a few billion years. Looking at the curves in the plot then a cold exoplanet with $i \sim 30^{\circ}$ might be comparable in size to the star. Unfortunately, the exoplanet doesn't transit so no radius measurement is available and it is unlikely to be inflated by stellar insolation because it is in a relatively wide orbit around a faint star

The answer depends on whether you mean is any planet bigger than any star (Case 1), or whether the planet and star have to be in the same system and have been discovered/measured (Case 2), rather than just that they could exist in principle.

The answer to Case 1 is yes - planets can be demonstrably bigger than stars. The answer to Case 2 is that whilst such systems may exist in principle, they are probably rare and there are not yet (in late 2024) any reported examples.

Case 1

Case 2

If one demands that the exoplanet and star are part of the same system, then although they could exist in principle (as per the discussion above), there aren't any examples (yet). The curves in the plot above are not dependent on the type of star a planet orbits. Therefore, in principle, it might be possible for a $>1M_J$ planet to be found orbiting a (only just) smaller $<0.1 M_\odot$ star, even if it receives negligible insolation.

In practice, giant exoplanets appear to be rare around low mass stars, so it could be some time before an example is found. A compilation of 18 giant exoplanets that transit M-dwarfs, and hence having measured radii, contains none that are likely to be bigger than their host star (Kanodia et al. 2024).

Using the radial velocity technique, a close candidate might be GJ3512b which is an exoplanet with $M\sin i = 0.46 M_{\rm Jup}$ (i.e. this is a minimum mass, since the orbital inclination $i<90^{\circ}$) that orbits an M5.5V star quite similar to Proxima Cen (Morales et al. 2019). The star has an estimated radius of $(0.139 \pm 0.005) R_\odot$ and the age is thought to be a few billion years. Looking at the curves in the plot then a cold exoplanet with $i <=30^{\circ}$, and thus having $M >= M_{\rm Jup}$, might be comparable in size to the star. Unfortunately, the exoplanet doesn't transit so no radius measurement is available and it is unlikely to be inflated by stellar insolation because it is in a relatively wide orbit around a faint star

In practice, giant exoplanets are rare around low mass stars, so it could be some time before an example is found. However, a close candidate might be GJ3512b which is an exoplanet with $M\sin i = 0.46 M_{\rm Jup}$ (i.e. this is a minimum mass, since the orbital inclination $i<90^{\circ}$) that orbits an M5.5V star quite similar to Proxima Cen (Morales et al. 2019). The star has an estimated radius of $(0.139 \pm 0.005) R_\odot$ and the age is thought to be a few billion years. Looking at the curves in the plot then a cold exoplanet with $i \sim 30^{\circ}$ might be comparable in size to the star. Unfortunately, the exoplanet doesn't transit so no radius measurement is available and it is unlikely to be inflated by stellar insolation because it is in a relatively wide orbit around a faint star

In practice, giant exoplanets are rare around low mass stars, so it could be some time before an example is found. However, a close candidate might be GJ3512b which is an exoplanet with $M\sin i = 0.46 M_{\rm Jup}$ (i.e. this is a minimum mass, since the orbital inclination $i<90^{\circ}$) that orbits an M5.5V star quite similar to Proxima Cen (Morales et al. 2019). The star has an estimated radius of $(0.139 \pm 0.005) R_\odot$ and the age is thought to be a few billion years. Looking at the curves in the plot then a cold exoplanet with $i \sim 30^{\circ}$ might be comparable in size to the star. Unfortunately, the exoplanet doesn't transit so no radius measurement is available and it is unlikely to be inflated by stellar insolation because it is in a relatively wide orbit around a faint star