<p>Beyond red dwarfs, another possibility is that of a planet orbiting a <a href="https://en.wikipedia.org/wiki/Subdwarf_B_star" rel="nofollow noreferrer">type B subdwarf star</a>. </p>

<p>Some features of such stars:</p>

<ul>
<li>Composed almost entirely of helium</li>
<li>Thought to be formed through the merger of two white dwarfs or at a specific point in the evolution of some red giants</li>
<li>Temperatures range from 20,000 K to 40,000 K</li>
<li>Brightness is between 10 - 100 times the brightness of the Sun</li>
<li>Mass is typically ~0.5 times the mass of the Sun</li>
<li><strong>Radius is around 0.15-0.25 times the radius of the sun</strong></li>
</ul>

<p>This radius range puts it in overlap of the radius of the largest planets (~0.2 times solar radius). Since the progenitor star(s) is more massive, it leads to an increased likelihood of gas giants forming in the protoplanetary disk. 
The question then becomes: "Can a gas giant find its way to the inner star system so that it is able to puff up?"</p>

<p>Two type-B Subdwarf stars with planets are known. 
<a href="https://en.wikipedia.org/wiki/V391_Pegasi" rel="nofollow noreferrer">V391 Pegasi</a> is perhaps the closest to fulfilling the planet-larger-than-star criterion. Orbiting the star at ~1.7 AU is a 2.5 - 3.99 <span class="math-container">$M_j$</span> gas giant. This gas giant will receive more energy from its star per square metre than the Earth does from the sun, but this is not likely enough for the planet to heat up sufficiently to become sufficiently 'puffy' to overtake the star's size of 2.3 <span class="math-container">$R_j$</span>.</p>

<p>The other known example is <a href="https://en.wikipedia.org/wiki/Kepler-70" rel="nofollow noreferrer">Kepler-70</a>, a rather curious star that appears to be the remnant of a red giant. The Kepler 70 system is very compact, with the two small (sub-earth radius) planets orbiting with a blisteringly-fast period of 5 and 8 hours respectively. (Fascinatingly, these planets weren't detected by eclipsing their host star, but rather by the periodic <em>increase</em> in luminosity as they begin to orbit behind the star. Both of these planets have surfaces hotter than the Sun, 7,600 K and 6,800 K respectively.) These planets are theorised to be the remnants of gas giants who were evaporated by being inside the star during its red giant phase.</p>

<p>From these tow examples, I conclude that there is no difficulty in having gas giants around small type-B subdwarf stars, although the mechanisms for bringing them close enough to become puffy planets is fraught with problems. You either have a red giant that boils all nearby gas giants before the subdwarf forms, or you have two white dwarfs that merge into a blue subdwarf, which requires a progenitor system of two close-binary stars that prohibit close-orbiting circumbinary planets. </p>

<p>I suspect for a planet-larger-than-host-star system to form, the gas giant has to migrate inwards somehow after the formation of the subdwarf star.</p>

CC BY-SA 4.0
2019-03-11T19:07:12.163Z
Alas, the tight error bars on the wikipedia article gave false confidence on the certainty of these planets
Ingolifs
CC BY-SA 4.0
2019-03-07T23:11:04.433Z
2019-03-07T23:11:04.433Z
Ingolifs
<p>The answer to the question depends on the exact definition of planet that is used.</p>

<p>A possible example is the L dwarf 2M 0746+20 (2MASS J07464256+2000321) and its planet <a href="http://exoplanet.eu/catalog/2m_0746+20_b/" rel="noreferrer">2M 0746+20 b</a>.</p>

<p>The radius of the planet is 12% greater than the radius of the star.</p>

<p><span class="math-container">$$\begin{array}{lll}
\hline
\text{} &amp; \text{Mass} &amp; \text{Radius}\\
\hline
\text{Planet} &amp; 12.21 \cdot M_J &amp; 0.970 \cdot R_J\\
\text{Star} &amp; 83.79 \cdot M_J &amp; 0.089 \cdot R_{Sun} = 0.866 \cdot R_J\\
\hline
\end{array}$$</span></p>

<p>Note: The reported planet mass of <span class="math-container">$12.21 (± 0.4) \cdot M_J$</span> is slightly below the deuterium burning limit of 13 Jupiter masses.</p>

CC BY-SA 4.0
2019-03-10T23:18:49.320Z
2019-03-10T23:18:49.320Z
aventurin
<p>The mass region of objects between ~ 0.5 Jupiter masses and 80 Jupiter masses (gas giants through to brown dwarfs and red dwarfs) is typified by an almost flat relationship with object diameter. There are planets out there which are larger in diameter than some of the smallest stars.</p>
<p>The smallest (currently fusing) star known, <a href="https://en.wikipedia.org/wiki/EBLM_J0555-57" rel="nofollow noreferrer">EBLM-J0555-57</a>, is estimated to be slightly larger than Saturn (at about 59000 km radius with 85 times Jupiter's mass).</p>
<p>One of the largest planets known that isn't a suspected brown dwarf, <a href="https://en.wikipedia.org/wiki/WASP-79b" rel="nofollow noreferrer">WASP-79b</a> is estimated to be twice Jupiter's diameter at 0.9 times Jupiter's mass. Many <a href="https://en.wikipedia.org/wiki/Hot_Jupiter" rel="nofollow noreferrer">Hot jupiters and puffy planets</a> with similar measurements are known.</p>
<p><strong>How likely are there to be systems where a planet is larger in diameter than its host star? Are there any examples known?</strong></p>
<p>I am looking for currently fusing stars only, which rules out pulsar planets, etc.</p>

CC BY-SA 4.0
2019-03-07T15:59:45.920Z
Are you going purely on mass, or would you go by radius, allowing a "young" planet whose gas or dust field is still going thru the coalescence process?  (not that I have any idea how to find those)
Carl Witthoft
CC BY-SA 4.0
2019-03-07T20:09:05.527Z
It has to be by radius, as stars are always more massive than planets.
Ingolifs
CC BY-SA 4.0
2022-05-30T06:51:01.887Z
If you include brown dwarfs there is a tiny overlap where the star can weight slightly less.
Kevin Kostlan
<planetary-formation><size><brown-dwarf><red-dwarf><hot-jupiter>
Existence of planets larger than their host star?
Ingolifs
# Existence of planets larger than their host star?

(Asked by: username_0 on 2019-03-06)

The mass region of objects between ~ 0.5 Jupiter masses and 80 Jupiter masses (gas giants through to brown dwarfs and red dwarfs) is typified by an almost flat relationship with object diameter. There are planets out there which are larger in diameter than some of the smallest stars.

The smallest (currently fusing) star known, [EBLM-J0555-57](https://en.wikipedia.org/wiki/EBLM_J0555-57), is estimated to be slightly larger than Saturn (at about 59000 km radius with 85 times Jupiter's mass).

One of the largest planets known that isn't a suspected brown dwarf, [WASP-79b](https://en.wikipedia.org/wiki/WASP-79b) is estimated to be twice Jupiter's diameter at 0.9 times Jupiter's mass. Many [Hot jupiters and puffy planets](https://en.wikipedia.org/wiki/Hot_Jupiter) with similar measurements are known.

**How likely are there to be systems where a planet is larger in diameter than its host star? Are there any examples known?**

I am looking for currently fusing stars only, which rules out pulsar planets, etc.

**Tags:** planetary-formation, size, brown-dwarf, red-dwarf, hot-jupiter
> **Comments:**
> **username_1**: Are you going purely on mass, or would you go by radius, allowing a "young" planet whose gas or dust field is still going thru the coalescence process?  (not that I have any idea how to find those)  
> **username_0**: It has to be by radius, as stars are always more massive than planets.  
> **username_2**: If you include brown dwarfs there is a tiny overlap where the star can weight slightly less.  

---

## 2 answers

### Answer by username_3 on 2019-03-10. Score: 9 | Accepted answer

The answer to the question depends on the exact definition of planet that is used.

A possible example is the L dwarf 2M 0746+20 (2MASS J07464256+2000321) and its planet [2M 0746+20 b](http://exoplanet.eu/catalog/2m_0746+20_b/).

The radius of the planet is 12% greater than the radius of the star.

$$\begin{array}{lll}
\hline
\text{} & \text{Mass} & \text{Radius}\\
\hline
\text{Planet} & 12.21 \cdot M\_J & 0.970 \cdot R\_J\\
\text{Star} & 83.79 \cdot M\_J & 0.089 \cdot R\_{Sun} = 0.866 \cdot R\_J\\
\hline
\end{array}$$

Note: The reported planet mass of $12.21 (± 0.4) \cdot M\_J$ is slightly below the deuterium burning limit of 13 Jupiter masses.


---

### Answer by username_0 on 2019-03-07. Score: 4

Beyond red dwarfs, another possibility is that of a planet orbiting a [type B subdwarf star](https://en.wikipedia.org/wiki/Subdwarf_B_star).

Some features of such stars:

* Composed almost entirely of helium
* Thought to be formed through the merger of two white dwarfs or at a specific point in the evolution of some red giants
* Temperatures range from 20,000 K to 40,000 K
* Brightness is between 10 - 100 times the brightness of the Sun
* Mass is typically ~0.5 times the mass of the Sun
* **Radius is around 0.15-0.25 times the radius of the sun**

This radius range puts it in overlap of the radius of the largest planets (~0.2 times solar radius). Since the progenitor star(s) is more massive, it leads to an increased likelihood of gas giants forming in the protoplanetary disk.
The question then becomes: "Can a gas giant find its way to the inner star system so that it is able to puff up?"

Two type-B Subdwarf stars with planets are known.
[V391 Pegasi](https://en.wikipedia.org/wiki/V391_Pegasi) is perhaps the closest to fulfilling the planet-larger-than-star criterion. Orbiting the star at ~1.7 AU is a 2.5 - 3.99 $M\_j$ gas giant. This gas giant will receive more energy from its star per square metre than the Earth does from the sun, but this is not likely enough for the planet to heat up sufficiently to become sufficiently 'puffy' to overtake the star's size of 2.3 $R\_j$.

The other known example is [Kepler-70](https://en.wikipedia.org/wiki/Kepler-70), a rather curious star that appears to be the remnant of a red giant. The Kepler 70 system is very compact, with the two small (sub-earth radius) planets orbiting with a blisteringly-fast period of 5 and 8 hours respectively. (Fascinatingly, these planets weren't detected by eclipsing their host star, but rather by the periodic *increase* in luminosity as they begin to orbit behind the star. Both of these planets have surfaces hotter than the Sun, 7,600 K and 6,800 K respectively.) These planets are theorised to be the remnants of gas giants who were evaporated by being inside the star during its red giant phase.

From these tow examples, I conclude that there is no difficulty in having gas giants around small type-B subdwarf stars, although the mechanisms for bringing them close enough to become puffy planets is fraught with problems. You either have a red giant that boils all nearby gas giants before the subdwarf forms, or you have two white dwarfs that merge into a blue subdwarf, which requires a progenitor system of two close-binary stars that prohibit close-orbiting circumbinary planets.

I suspect for a planet-larger-than-host-star system to form, the gas giant has to migrate inwards somehow after the formation of the subdwarf star.

> **Comments:**
> **username_0**: Alas, the tight error bars on the wikipedia article gave false confidence on the certainty of these planets  

---

