<p>The answer depends on whether you mean is <em>any</em> planet bigger than <em>any</em> 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.</p>
<p>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.</p>
<p><strong>Case 1</strong></p>
<p>There <em>are</em> a few known planets with measured radii that are bigger than the lowest mass stars.
Here is a plot from <a href="https://www.semanticscholar.org/paper/The-mass%E2%80%90radius-relationship-from-solar%E2%80%90type-stars-Chabrier-Baraffe/413fa4a519b6442ad1e1bfe147af2322fec827d7" rel="nofollow noreferrer">Chabrier et al. (2008)</a> (and plenty more data will have been added since), which shows the basic picture. This is the mass-radius plot for both stars and exoplanets.
<a href="https://i.sstatic.net/lSozb.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/lSozb.png" alt="Mass-radius plot for stars and exoplanets" /></a></p>
<p>It turns out that there are some hot Jupiters that have radii about twice that of Jupiter in our Solar System. You can find examples at <a href="http://exoplanets.org/" rel="nofollow noreferrer">exoplanets.org</a>, such as HAT P-67b and XO-6b. These planets are bigger than theory suggests for a &quot;cold&quot; exoplanet, probably because of &quot;insolation&quot; (heating by their parent star) - e.g. <a href="https://arxiv.org/abs/1202.6199" rel="nofollow noreferrer">Enoch et al. (2012)</a>.</p>
<p>On the other hand, the smallest stars, those just above the brown dwarf limit of <span class="math-container">$\sim 0.075M_\odot$</span>, that are predicted to have radii (at least once they are a billion years old and have reached the main sequence), of about 1.3 times that of Jupiter. At older ages they can become even smaller - about the size of Saturn (black dashed line).</p>
<p>In terms of measurements, there are low-mass objects in eclipsing binaries and also a handful of very low-mass stars that have interferometric radii. For example Proxima Cen (the nearest star to the Sun) is reported to have an interferometric radius of <span class="math-container">$(0.145 \pm 0.011)R_\odot$</span> (or 1.44 Jupiter radii) by <a href="https://www.aanda.org/articles/aa/full_html/2009/37/aa11976-09/aa11976-09.html" rel="nofollow noreferrer">Demory et al. (2009)</a> and so this is clearly smaller than the biggest exoplanets.</p>
<p><strong>Case 2</strong></p>
<p>If one demands that the exoplanet and star are part of the <em>same</em> 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 <span class="math-container">$&gt;1M_J$</span> planet to be found orbiting a (only just) <em>smaller</em> <span class="math-container">$&lt;0.1 M_\odot$</span> star, even if it receives negligible insolation.</p>
<p>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 (<a href="https://iopscience.iop.org/article/10.3847/1538-3881/ad27cb" rel="nofollow noreferrer">Kanodia et al. 2024</a>).</p>
<p>Using the radial velocity technique, a close <em>candidate</em> might be GJ3512b which is an exoplanet with <span class="math-container">$M\sin i = 0.46 M_{\rm Jup}$</span> (i.e. this is a <em>minimum</em> mass, since the orbital inclination <span class="math-container">$i&lt;90^{\circ}$</span>) that orbits an M5.5V star quite similar to Proxima Cen (<a href="https://arxiv.org/abs/1909.12174" rel="nofollow noreferrer">Morales et al. 2019</a>). The star has an estimated radius of <span class="math-container">$(0.139 \pm 0.005) R_\odot$</span> and the age is thought to be a few billion years. Looking at the curves in the plot then a cold exoplanet with <span class="math-container">$i &lt;=30^{\circ}$</span>, and thus having <span class="math-container">$M &gt;= M_{\rm Jup}$</span>, 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</p>
<p>An interesting suggestion is that a <em>young</em> 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 <span class="math-container">$0.1$</span> 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 &quot;exoplanets&quot; (<span class="math-container">$&gt;5$</span> Jupiter masses) orbiting at very large distances (<span class="math-container">$&gt;100$</span> au) are more like binary brown dwarfs.</p>
<p><strong>Case 3</strong></p>
<p>There are now examples of planets that orbits compact stars (neutron stars and brown dwarfs). In fact some of the first exoplanets discovered were <a href="https://en.wikipedia.org/wiki/Pulsar_planet" rel="nofollow noreferrer">around pulsars</a>. Even the least massive of these &quot;pulsar planets&quot; is likely to be bigger than the <span class="math-container">$\sim$</span>10 km radius of the neutron star it orbits.</p>
<p>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.</p>
<p>JWST has also directly imaged two planetary candidates around two white dwarfs (<a href="https://iopscience.iop.org/article/10.3847/2041-8213/ad2348" rel="nofollow noreferrer">Mullally et al. 2024</a>). 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.</p>

CC BY-SA 4.0
2021-10-27T13:55:18.147Z
I have played around a little bit with the numbers to see if a specific planet with a specific host star can be larger. I.e. if we want to inflate a 'Hot' Jupiter orbiting the smallest M dwarf, $M=0.08M_{\odot}$ and assuming it sits on its main sequence, we get $R=0.1R_{\odot}$ and $T_{\rm eff}=2000 K$, then the incident flux  when orbiting at 3 $R_{J}$ distance is about $\rm9\times 10^{7} erg cm^{-2}s^{-1}$, which is just below the empirical threshold value for HJ inflation ($\rm2\times 10^{8} erg cm^{-2}s^{-1}$). Now this is only the minimum stellar mass, but chances for $R_p>R_s$ seem slim.
AtmosphericPrisonEscape
CC BY-SA 4.0
2021-10-27T17:19:24.830Z
@AtmosphericPrisonEscape as you can see from the plot, "uninflated" planets of more than one Jupiter mass should be bigger than $0.1 R_\odot$ (indeed, Jupiter is, just).
ProfRob
CC BY-SA 4.0
2021-10-27T19:27:33.560Z
Yes, but that is for *any* planet and star found. The plotted planets and stars are not orbiting pairs. I asked the question whether there is an actual star-planet combination that can fulfill the requirement to inflate the planet via the large L condition *and* fulfill $R_{p}>R_{s}$.
AtmosphericPrisonEscape
CC BY-SA 4.0
2021-10-27T19:32:38.757Z
@AtmosphericPrisonEscape the relationship between the mass and size of a cold planet (the solid line in the picture) is independent of the type of star the planet orbits. Therefore there is no reason in principle that a very low mass star should not have a $>1M_J$ planet that is bigger than it. In practice, giant planets around low mass stars are rare.
ProfRob
CC BY-SA 4.0
2021-10-27T20:24:16.250Z
"Therefore there is no reason in principle that a very low mass star should not have a >1MJ planet that is bigger than it" Which is what I was investigating.
AtmosphericPrisonEscape
CC BY-SA 4.0
2021-10-28T15:46:20.780Z
One possible solution would be a recently formed and therefore hot Hupiter, or one that had just collided with a terresital planet.
blademan9999
CC BY-SA 4.0
2021-10-28T15:52:50.393Z
@blademan9999 Possibly, although if it is young enough to be much bigger then it is likely that the star is also bigger (pre main sequence). But then the contraction timescale for the planet *is* probably longer than the star.
ProfRob
CC BY-SA 4.0
2021-10-27T11:08:45.183Z
2024-11-29T12:34:38.663Z
2024-11-29T12:34:38.663Z
ProfRob
<p>If by &quot;bigger&quot; you are referring to <strong>mass</strong>, disregarding radius, then the answer is <strong>strictly no</strong> for a regular &quot;star&quot;</p>
<p>A star is a body that has reached a stable state in which gravitational collapse is balanced by nuclear fusion in its core.  A &quot;planet&quot; that was bigger (more massive) than a star would have a core that was hot and dense enough for fusion to begin, and so would be a star.  A star that had less mass than a planet would not have a core that was hot and dense enough for fusion to start, and so would not be a star!</p>
<p>There is an exception: if you are willing to accept stellar remnants like white dwarfs and neutron stars as &quot;stars&quot; (they are hot but there is no fusion occurring) then these can be smaller (in diameter) but much more massive (weight) than a planet.</p>
<p>For convenience we normally consider bodies that are up to 13 times the mass of Jupiter to be &quot;planets&quot;, 13-80 times the mass of Jupiter to be &quot;brown dwarfs&quot; (they don't have significant hydrogen fusion in their cores, but do have some deuterium and perhaps lithium fusion) and over 80 times the mass of Jupiter are &quot;stars&quot;.  By this definition &quot;stars&quot; must be more massive than planets.</p>
<p>A white dwarf would be about the size of a moon or small planet, but a mass of about 200-1300 times the mass of Jupiter.  Neutron stars are even more extreme, with a size comparable to an asteroid, but a mass of several thousand Jupiters.</p>
<p>There are planets that are known to orbit around neutron stars.  The planet would be physically larger than the star, but much much less massive. An example is <a href="https://en.wikipedia.org/wiki/PSR_B1257%2B12" rel="nofollow noreferrer">PSR B1257+B12</a>. The neutron star is about 10km in radius and has mass greater than the sun. Its planets are much less massive, but very roughly Earth sized.</p>

CC BY-SA 4.0
2021-10-27T22:41:59.217Z
That contradicts Prof Rob's answer, which has actual examples.  Hot Jupiters vs Red Dwarfs, which are "regular stars" (not brown dwarfs).
JDługosz
CC BY-SA 4.0
2021-10-28T12:09:35.263Z
@JDługosz such is the problem with accepting questions that use vague terms like "bigger" :D
Caius Jard
CC BY-SA 4.0
2021-10-28T14:43:21.620Z
"bigger" is a different word than "heavier".  What's vague about it?
JDługosz
CC BY-SA 4.0
2021-10-28T15:26:28.407Z
@JDługosz, what's *not* vague about it? "Bigger" could refer either to bigger radius / volume or to bigger mass.  Both senses are fairly common. Indeed, there is a highly upvoted comment on the question requesting exactly this clarification, to which the OP has not yet responded.
John Bollinger
CC BY-SA 4.0
2021-10-28T20:38:13.787Z
@JohnBollinger Would you really say that a cardboard box filled with stuff is _bigger_ than the same box when empty?  I think that the speaker talking about a "bigger" object may implicitly refer to their masses and that the bigger one is also heavier, but the calling of one _bigger_ is keying off the visible shape.  If the two properties were not varying in the same direction, one would point out that "the bigger one is lighter".  I submit that a native speaker would never call a smaller but heavier choice "bigger".
JDługosz
CC BY-SA 4.0
2021-10-28T22:08:13.230Z
@JDługosz, I would not say that of a cardboard box, but I very well might, and others definitely do, say it of a planet or star.  Context matters.  The fact that a clarification on this matter was requested on the question and was so highly upvoted should stand as evidence that lots of people do see an ambiguity here.  I grant that using "bigger" in astronomical context to mean "more massive" can be viewed as imprecise, but it is nevertheless relatively common in such context.
John Bollinger
CC BY-SA 4.0
2021-10-30T06:16:46.390Z
@JohnBollinger I agree. Context matters and the cardboards box example is contrived. The same kind of ambiguity reappears when talking about countries. Those can be bigger by area or by population and both are completely valid.
Raphael Schweikert
CC BY-SA 4.0
2021-10-27T08:00:04.697Z
2021-10-31T08:00:44.610Z
2021-10-31T08:00:44.610Z
James K
<p>The red dwarf EBLM J0555-57Ab is smaller than Saturn.</p>
<p><a href="https://en.wikipedia.org/wiki/EBLM_J0555-57" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/EBLM_J0555-57</a></p>

CC BY-SA 4.0
2021-10-29T10:48:25.260Z
2021-10-29T10:48:25.260Z
sno
<p>Just from theoretical principles, suppose there is some mass <span class="math-container">$M_s$</span> above which a body undergoes gravitational collapse and becomes a star, and below which it doesn't. A body with mass <span class="math-container">$M_s+\epsilon$</span> would become a star, begin fusion, then radiate energy, becoming a body with mass less than <span class="math-container">$M_s$</span>. However, it would continue being a star; once a body has undergone gravitational collapse, its density increases, and the mass needed to sustain gravitational collapse is lower. There would need to be a certain amount of energy radiated away before the body ceases being able to sustain fusion. So we should expect there to be a range from <span class="math-container">$M_s$</span> to whatever this lower mass is in which there exist both planets and stars.</p>
<p>On top of this, having a sharp boundary of <span class="math-container">$M_s$</span> would depend on all star candidates of the same mass being the same in terms of density, composition, temperature, etc. Variations in such characteristics would be another factor creating a &quot;star or planet&quot; band of mass.</p>

CC BY-SA 4.0
2021-10-29T19:06:43.140Z
2021-10-29T19:06:43.140Z
Acccumulation
<p>Or a star smaller than a planet?</p>
<p>Which star and planet would be an example of this?</p>

CC BY-SA 4.0
2021-10-27T02:59:08.477Z
https://www.usatoday.com/story/tech/science/2017/07/11/smallest-star-universe-has-been-discovered/469773001/
Keith McClary
CC BY-SA 4.0
2021-10-27T13:11:54.117Z
Do neutron stars count as "stars" for this?
TimRias
CC BY-SA 4.0
2021-10-27T15:37:18.047Z
By "bigger" are you referring to volume or mass?
Boluc Papuccuoglu
CC BY-SA 4.0
2021-10-27T19:05:13.710Z
@mmeent white dwarfs also qualify. The heaviest of them are smaller than the Moon.
fraxinus
CC BY-SA 4.0
2021-10-27T20:36:14.667Z
Do brown dwarves count as stars or planets?  Or neither?
RBarryYoung
CC BY-SA 4.0
2021-10-27T21:42:44.627Z
Have you not heard heard of dwarf stars, or even black holes?
Robbie Goodwin
CC BY-SA 4.0
2021-10-28T00:56:01.460Z
@fraxinus "white dwarfs also qualify" There's no fusion in white dwarfs, so why would they qualify?
Don Branson
CC BY-SA 4.0
2021-10-28T06:00:36.800Z
@DonBranson mostly... https://www.nasa.gov/feature/goddard/2021/hubble-discovers-hydrogen-burning-white-dwarfs-enjoying-slow-aging
fraxinus
CC BY-SA 4.0
2021-10-28T07:47:37.293Z
Is there any planet bigger than a star where the star is not smaller than the planet? Which star and planet would be an example of this?
CJ Dennis
CC BY-SA 4.0
2021-10-28T11:38:58.903Z
Bigger than *a* star or bigger than *their* star?
Mast
CC BY-SA 4.0
2021-10-29T00:27:24.450Z
It seems like one can fairly easily answer this oneself by looking up [the largest planets](https://en.wikipedia.org/wiki/List_of_largest_exoplanets) and then looking up [the smallest stars](https://en.wikipedia.org/wiki/List_of_smallest_stars).
NotThatGuy
CC BY-SA 4.0
2021-11-29T10:50:06.567Z
Only on first thought a bollocks question :)
planetmaker
<star><exoplanet><size>
Is there any planet bigger than a star?
asker223
# Is there any planet bigger than a star?

(Asked by: username_0 on 2021-10-27)

Or a star smaller than a planet?

Which star and planet would be an example of this?

**Tags:** star, exoplanet, size

> **Comments:**
> **username_1**: https://www.usatoday.com/story/tech/science/2017/07/11/smallest-star-universe-has-been-discovered/469773001/  
> **username_2**: Do neutron stars count as "stars" for this?  
> **username_3**: By "bigger" are you referring to volume or mass?  
> **username_4**: @mmeent white dwarfs also qualify. The heaviest of them are smaller than the Moon.  
> **username_5**: Do brown dwarves count as stars or planets?  Or neither?  
> **username_6**: Have you not heard heard of dwarf stars, or even black holes?  
> **username_7**: @username_4 "white dwarfs also qualify" There's no fusion in white dwarfs, so why would they qualify?  
> **username_4**: @DonBranson mostly... https://www.nasa.gov/feature/goddard/2021/hubble-discovers-hydrogen-burning-white-dwarfs-enjoying-slow-aging  
> **username_8**: Is there any planet bigger than a star where the star is not smaller than the planet? Which star and planet would be an example of this?  
> **username_9**: Bigger than *a* star or bigger than *their* star?  
> **username_10**: It seems like one can fairly easily answer this oneself by looking up [the largest planets](https://en.wikipedia.org/wiki/List_of_largest_exoplanets) and then looking up [the smallest stars](https://en.wikipedia.org/wiki/List_of_smallest_stars).  
> **username_11**: Only on first thought a bollocks question :)  

---

## 4 answers

### Answer by username_12 on 2021-10-27. Score: 54

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.

**Case 1**

There *are* a few known planets with measured radii that are bigger than the lowest mass stars.
Here is a plot from [Chabrier et al. (2008)](https://www.semanticscholar.org/paper/The-mass%E2%80%90radius-relationship-from-solar%E2%80%90type-stars-Chabrier-Baraffe/413fa4a519b6442ad1e1bfe147af2322fec827d7) (and plenty more data will have been added since), which shows the basic picture. This is the mass-radius plot for both stars and exoplanets.
[![Mass-radius plot for stars and exoplanets](https://i.sstatic.net/lSozb.png)](https://i.sstatic.net/lSozb.png)

It turns out that there are some hot Jupiters that have radii about twice that of Jupiter in our Solar System. You can find examples at [exoplanets.org](http://exoplanets.org/), such as HAT P-67b and XO-6b. These planets are bigger than theory suggests for a "cold" exoplanet, probably because of "insolation" (heating by their parent star) - e.g. [Enoch et al. (2012)](https://arxiv.org/abs/1202.6199).

On the other hand, the smallest stars, those just above the brown dwarf limit of $\sim 0.075M\_\odot$, that are predicted to have radii (at least once they are a billion years old and have reached the main sequence), of about 1.3 times that of Jupiter. At older ages they can become even smaller - about the size of Saturn (black dashed line).

In terms of measurements, there are low-mass objects in eclipsing binaries and also a handful of very low-mass stars that have interferometric radii. For example Proxima Cen (the nearest star to the Sun) is reported to have an interferometric radius of $(0.145 \pm 0.011)R\_\odot$ (or 1.44 Jupiter radii) by [Demory et al. (2009)](https://www.aanda.org/articles/aa/full_html/2009/37/aa11976-09/aa11976-09.html) and so this is clearly smaller than the biggest exoplanets.

**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](https://iopscience.iop.org/article/10.3847/1538-3881/ad27cb)).

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](https://arxiv.org/abs/1909.12174)). 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

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](https://en.wikipedia.org/wiki/Pulsar_planet). 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](https://iopscience.iop.org/article/10.3847/2041-8213/ad2348)). 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.

> **Comments:**
> **username_13**: I have played around a little bit with the numbers to see if a specific planet with a specific host star can be larger. I.e. if we want to inflate a 'Hot' Jupiter orbiting the smallest M dwarf, $M=0.08M_{\odot}$ and assuming it sits on its main sequence, we get $R=0.1R_{\odot}$ and $T_{\rm eff}=2000 K$, then the incident flux  when orbiting at 3 $R_{J}$ distance is about $\rm9\times 10^{7} erg cm^{-2}s^{-1}$, which is just below the empirical threshold value for HJ inflation ($\rm2\times 10^{8} erg cm^{-2}s^{-1}$). Now this is only the minimum stellar mass, but chances for $R_p>R_s$ seem slim.  
> **username_12**: @username_13 as you can see from the plot, "uninflated" planets of more than one Jupiter mass should be bigger than $0.1 R_\odot$ (indeed, Jupiter is, just).  
> **username_13**: Yes, but that is for *any* planet and star found. The plotted planets and stars are not orbiting pairs. I asked the question whether there is an actual star-planet combination that can fulfill the requirement to inflate the planet via the large L condition *and* fulfill $R_{p}>R_{s}$.  
> **username_12**: @username_13 the relationship between the mass and size of a cold planet (the solid line in the picture) is independent of the type of star the planet orbits. Therefore there is no reason in principle that a very low mass star should not have a $>1M_J$ planet that is bigger than it. In practice, giant planets around low mass stars are rare.  
> **username_13**: "Therefore there is no reason in principle that a very low mass star should not have a >1MJ planet that is bigger than it" Which is what I was investigating.  
> **username_14**: One possible solution would be a recently formed and therefore hot Hupiter, or one that had just collided with a terresital planet.  
> **username_12**: @username_14 Possibly, although if it is young enough to be much bigger then it is likely that the star is also bigger (pre main sequence). But then the contraction timescale for the planet *is* probably longer than the star.  

---

### Answer by username_15 on 2021-10-27. Score: 10

If by "bigger" you are referring to **mass**, disregarding radius, then the answer is **strictly no** for a regular "star"

A star is a body that has reached a stable state in which gravitational collapse is balanced by nuclear fusion in its core. A "planet" that was bigger (more massive) than a star would have a core that was hot and dense enough for fusion to begin, and so would be a star. A star that had less mass than a planet would not have a core that was hot and dense enough for fusion to start, and so would not be a star!

There is an exception: if you are willing to accept stellar remnants like white dwarfs and neutron stars as "stars" (they are hot but there is no fusion occurring) then these can be smaller (in diameter) but much more massive (weight) than a planet.

For convenience we normally consider bodies that are up to 13 times the mass of Jupiter to be "planets", 13-80 times the mass of Jupiter to be "brown dwarfs" (they don't have significant hydrogen fusion in their cores, but do have some deuterium and perhaps lithium fusion) and over 80 times the mass of Jupiter are "stars". By this definition "stars" must be more massive than planets.

A white dwarf would be about the size of a moon or small planet, but a mass of about 200-1300 times the mass of Jupiter. Neutron stars are even more extreme, with a size comparable to an asteroid, but a mass of several thousand Jupiters.

There are planets that are known to orbit around neutron stars. The planet would be physically larger than the star, but much much less massive. An example is [PSR B1257+B12](https://en.wikipedia.org/wiki/PSR_B1257%2B12). The neutron star is about 10km in radius and has mass greater than the sun. Its planets are much less massive, but very roughly Earth sized.

> **Comments:**
> **username_16**: That contradicts Prof Rob's answer, which has actual examples.  Hot Jupiters vs Red Dwarfs, which are "regular stars" (not brown dwarfs).  
> **username_17**: @username_16 such is the problem with accepting questions that use vague terms like "bigger" :D  
> **username_16**: "bigger" is a different word than "heavier".  What's vague about it?  
> **username_18**: @username_16, what's *not* vague about it? "Bigger" could refer either to bigger radius / volume or to bigger mass.  Both senses are fairly common. Indeed, there is a highly upvoted comment on the question requesting exactly this clarification, to which the OP has not yet responded.  
> **username_16**: @JohnBollinger Would you really say that a cardboard box filled with stuff is _bigger_ than the same box when empty?  I think that the speaker talking about a "bigger" object may implicitly refer to their masses and that the bigger one is also heavier, but the calling of one _bigger_ is keying off the visible shape.  If the two properties were not varying in the same direction, one would point out that "the bigger one is lighter".  I submit that a native speaker would never call a smaller but heavier choice "bigger".  
> **username_18**: @username_16, I would not say that of a cardboard box, but I very well might, and others definitely do, say it of a planet or star.  Context matters.  The fact that a clarification on this matter was requested on the question and was so highly upvoted should stand as evidence that lots of people do see an ambiguity here.  I grant that using "bigger" in astronomical context to mean "more massive" can be viewed as imprecise, but it is nevertheless relatively common in such context.  
> **username_19**: @JohnBollinger I agree. Context matters and the cardboards box example is contrived. The same kind of ambiguity reappears when talking about countries. Those can be bigger by area or by population and both are completely valid.  

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### Answer by username_20 on 2021-10-29. Score: 2

The red dwarf EBLM J0555-57Ab is smaller than Saturn.

<https://en.wikipedia.org/wiki/EBLM_J0555-57>


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### Answer by username_21 on 2021-10-29. Score: 1

Just from theoretical principles, suppose there is some mass $M\_s$ above which a body undergoes gravitational collapse and becomes a star, and below which it doesn't. A body with mass $M\_s+\epsilon$ would become a star, begin fusion, then radiate energy, becoming a body with mass less than $M\_s$. However, it would continue being a star; once a body has undergone gravitational collapse, its density increases, and the mass needed to sustain gravitational collapse is lower. There would need to be a certain amount of energy radiated away before the body ceases being able to sustain fusion. So we should expect there to be a range from $M\_s$ to whatever this lower mass is in which there exist both planets and stars.

On top of this, having a sharp boundary of $M\_s$ would depend on all star candidates of the same mass being the same in terms of density, composition, temperature, etc. Variations in such characteristics would be another factor creating a "star or planet" band of mass.


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