<h2>Big picture:</h2>

<p>The reason why there are planets so close to their host star is probably <strong>planetary migration</strong>. These planets are difficult to explain by <em>in situ</em> formation models (first of all there is not enough material to form that kind of massive objects in these regions). There are different theories to explain planetary migration:</p>

<ul>
<li><strong>disk-planet interactions</strong>: planets form in a protoplanetary disk that still contains lots of gas after planet formation, and that can interact with the formed planets. There are different interactions that can explain migration, mostly depending on the mass of the planet you consider. We distinguish 3 main types of migration processes: <em>Type I</em> (migration of low-mass embedded planets), <em>Type II</em> (migration and gap formation by massive planets) and <em>Type III</em> migration (rapid migration of intermediate mass planet in relatively massive disk).</li>
<li><strong>planet-planet interactions</strong>: gravitational scattering can be also a very efficient process to modify planet orbits.</li>
<li><strong>Kozai mechanism</strong>: this mechanism happens in a three body problem. One object can be perturbated by an other object orbiting farther out; the object then starts to oscillate around its orbit, and can therefore migrates.</li>
<li><strong>Interaction with planetesimals</strong> (also refers as the <em>Nice model</em> for the Solar System): interaction with a remnant planetesimal disk (as the Kuiper disk for the Solar System) can cause migration. Planets can exchange angular momentum with planetesimals in the outer edge of the disk by gravitational interaction, and thus migrating.</li>
</ul>

<h2>Disk-planet interaction:</h2>

<p>Some more details about the <em>Type I, II and III</em> migration mechanisms:</p>

<ul>
<li><strong>Type I</strong>: planets are generating density waves propagating in the disk. Due to Keplerian rotation, this density waves become spiral waves, generating a <em>Lindblad torque</em>. The inner spiral arm is pulling the planet forward, the planet gaining angular momentum and migrating outward, whereas the outer spiral arm is is pulling the planet back and drives inward migration. In most cases, the outer Lindblad resonances take over the inner Lindblad resonances because they lie closer to the planet, and therefore drive an inward migration.</li>
<li><strong>Type II</strong>: massive planets open gaps in the disk in which they form. The gravitational interactions become stronger and thus the angular momentum transfer to the disk. If angular momentum is deposit locally in the disk, material inside the planet loses angular momentum and material outside gains angular momentum, and therefore it recedes from the planet, opening a gap in the disk. This gap reduces the importance of Lindblad torques, and the planet is then coupled to the viscous evolution of the disk. Since material from the disk is accreted on the star and angular momentum transported away, massive planets are migrating inward.</li>
<li><strong>Type III</strong>: if the surface density distribution in the co-orbital region of the planet is asymmetric, it can lead to a large torque that can cause the planet to fall inwards on a short timescale.</li>
</ul>

<p><strong>Sources:</strong></p>

<ul>
<li><a href="http://adsabs.harvard.edu/abs/2012ARA%26A..50..211K">Kley &amp; Nelson 2012</a></li>
</ul>

CC BY-SA 3.0
2013-10-23T18:24:07.467Z
Please see the update to the question... I don't see how most of the things mentioned here could migrate a large plant to be so close to the star, in what seems like a stable orbit.
Glen Little
CC BY-SA 3.0
2013-10-24T11:51:36.170Z
I'm not sure to see the point of your comment. I gave you six possible mechanisms that can cause migration. And there is no such thing as a stable orbit in the long run, as soon as you have 3 bodies or more in a gravitational system, it is a chaotic system, and the objects can migrate inward or outward. So first thing is: we observe planets very close to their host star, that can be explained only by migration. Then, they could probably migrate outward, as it happened in our Solar System.
MBR
CC BY-SA 3.0
2013-10-24T23:56:23.890Z
Thanks MBR. I like the information presented here, but I was looking for "accepted theories that successfully predicted" this before it was observed.  The 4 theories mentioned all try to explain it, but they did not require planets to be in such a close orbit. The only system we have reliable information about (so far) is our own, and as you say, the planets here are migrating outward.
Glen Little
CC BY-SA 3.0
2015-10-19T06:46:51.767Z
there is also selection bias, our previous methods of planet detection, gained higher weight for close to the star planets
Qbik
CC BY-SA 3.0
2013-10-23T09:36:26.037Z
2013-10-23T09:36:26.037Z
MBR
<p>Because more massive, closely-orbiting planet exert much greater gravitational forces on their host star than smaller, more distant planets, there is a <em>significant</em> observational bias. We are much more likely to detect these planets (i.e. "<a href="http://en.wikipedia.org/wiki/Hot_Jupiter" rel="nofollow noreferrer">hot Jupiters</a>") because their observable effects (Doppler wobble, gravitational lensing, etc) are more significant.</p>

<p>The below image (<a href="http://en.wikipedia.org/wiki/Exoplanet#Orbital_parameters" rel="nofollow noreferrer">from here</a>) shows you that we're still mostly unable to detect planets like those in our own Solar system (the gray circles are our Solar planets). It's just too hard.</p>

<p>I don't study planetary formation (yet!), so I can't speak well for the theoretical side of things. All we've managed to collect so far is the very lowest-hanging fruit, so it is highly likely that there are is a <a href="http://www.space.com/19659-alien-earth-exoplanets-red-dwarfs.html" rel="nofollow noreferrer">large number</a> of Earth-like planets out there too.</p>

<p>The authors of <a href="http://mnras.oxfordjournals.org/content/430/2/951" rel="nofollow noreferrer">this article</a> on the Kepler-10 system (you'll need MNRAS access) suggest planet-planet gravitational scattering or collision-merger events. The <a href="http://www.scholarpedia.org/article/Planetary_formation_and_migration" rel="nofollow noreferrer">Scholarpedia</a> article provides a bit more brief reading on these mechanisms. <a href="http://www.eos.ubc.ca/~mjelline/453website/eosc453/E_prints/newfer010/chambers_planetarymigration_AR09.pdf" rel="nofollow noreferrer">Here is a review article</a> from 2009,  and a more <a href="http://arxiv.org/abs/astro-ph/0510487" rel="nofollow noreferrer">in-depth review</a> from 2006.</p>

<p><img src="https://i.sstatic.net/XWp3B.png" alt="Exoplanets discovered up until 2010-10-03."></p>

CC BY-SA 3.0
2013-10-23T14:15:43.983Z
It does not really answer the question of the OP; you explain instead why we detect them. It is also interesting, but irrelevant here.
MBR
CC BY-SA 3.0
2013-10-23T18:28:35.160Z
Thanks. This is interesting too, showing how many are very close!
Glen Little
CC BY-SA 3.0
2013-10-23T18:30:00.887Z
Your link brought me to http://en.wikipedia.org/wiki/Planetary_migration which touches on this, but doesn't seem to mention any theory that predicted what we are seeing now.
Glen Little
CC BY-SA 3.0
2013-10-23T23:48:47.357Z
In essence, the OP is wondering why so many planets are being found very close to their host stars. My answer explains *why* so many are *found* close to the star. MBR explains, much better than I could, *how* they get there. I think that both answers work together. @GlenLittle - see my edited answer. I have found some more references that outline current theory.
Moriarty
CC BY-SA 3.0
2013-10-24T10:25:16.727Z
Yes, the problem is difference between "Are close to the star" and "Are observed" close to the star". There may be a vast number of planets distant from the stars and completely overshadowing the number that is close to the stars. We're just unable to detect them. It's like "Why are there so many more white cats out at night than black ones?" - no, the proportions are the same, you just don't see the black cats because they hide in darkness better.
SF.
CC BY-SA 3.0
2013-10-23T04:26:30.807Z
2013-10-23T23:48:05.430Z
2013-10-23T23:48:05.430Z
Moriarty
<blockquote>
  <p>"Kepler-10b orbits its parent star once every 0.84 days, which means
  it is more than 20 times closer to its star than Mercury is to our
  sun" <a href="http://www.space.com/10592-smallest-alien-planet-discovered-nasa.html" rel="noreferrer">space.com</a></p>
</blockquote>

<p>Are there any accepted theories that successfully predicted that so many planets would be found so close to their star?  </p>

<p><strong>Edit:</strong> This is especially a question, since it seems that planets are moving away from their star. <a href="http://www.newscientist.com/article/dn17228-why-is-the-earth-moving-away-from-the-sun.html#.UmgRDhAwfi0" rel="noreferrer">newscientist.com</a></p>

<exoplanet><orbit>
Why are some planets so close to their star?
Glen Little
# Why are some planets so close to their star?

(Asked by: username_0 on 2013-10-23)

> "Kepler-10b orbits its parent star once every 0.84 days, which means
> it is more than 20 times closer to its star than Mercury is to our
> sun" [space.com](http://www.space.com/10592-smallest-alien-planet-discovered-nasa.html)

Are there any accepted theories that successfully predicted that so many planets would be found so close to their star?

**Edit:** This is especially a question, since it seems that planets are moving away from their star. [newscientist.com](http://www.newscientist.com/article/dn17228-why-is-the-earth-moving-away-from-the-sun.html#.UmgRDhAwfi0)

**Tags:** exoplanet, orbit

---

## 2 answers

### Answer by username_1 on 2013-10-23. Score: 12

## Big picture:

The reason why there are planets so close to their host star is probably **planetary migration**. These planets are difficult to explain by *in situ* formation models (first of all there is not enough material to form that kind of massive objects in these regions). There are different theories to explain planetary migration:

* **disk-planet interactions**: planets form in a protoplanetary disk that still contains lots of gas after planet formation, and that can interact with the formed planets. There are different interactions that can explain migration, mostly depending on the mass of the planet you consider. We distinguish 3 main types of migration processes: *Type I* (migration of low-mass embedded planets), *Type II* (migration and gap formation by massive planets) and *Type III* migration (rapid migration of intermediate mass planet in relatively massive disk).
* **planet-planet interactions**: gravitational scattering can be also a very efficient process to modify planet orbits.
* **Kozai mechanism**: this mechanism happens in a three body problem. One object can be perturbated by an other object orbiting farther out; the object then starts to oscillate around its orbit, and can therefore migrates.
* **Interaction with planetesimals** (also refers as the *Nice model* for the Solar System): interaction with a remnant planetesimal disk (as the Kuiper disk for the Solar System) can cause migration. Planets can exchange angular momentum with planetesimals in the outer edge of the disk by gravitational interaction, and thus migrating.

## Disk-planet interaction:

Some more details about the *Type I, II and III* migration mechanisms:

* **Type I**: planets are generating density waves propagating in the disk. Due to Keplerian rotation, this density waves become spiral waves, generating a *Lindblad torque*. The inner spiral arm is pulling the planet forward, the planet gaining angular momentum and migrating outward, whereas the outer spiral arm is is pulling the planet back and drives inward migration. In most cases, the outer Lindblad resonances take over the inner Lindblad resonances because they lie closer to the planet, and therefore drive an inward migration.
* **Type II**: massive planets open gaps in the disk in which they form. The gravitational interactions become stronger and thus the angular momentum transfer to the disk. If angular momentum is deposit locally in the disk, material inside the planet loses angular momentum and material outside gains angular momentum, and therefore it recedes from the planet, opening a gap in the disk. This gap reduces the importance of Lindblad torques, and the planet is then coupled to the viscous evolution of the disk. Since material from the disk is accreted on the star and angular momentum transported away, massive planets are migrating inward.
* **Type III**: if the surface density distribution in the co-orbital region of the planet is asymmetric, it can lead to a large torque that can cause the planet to fall inwards on a short timescale.

**Sources:**

* [Kley & Nelson 2012](http://adsabs.harvard.edu/abs/2012ARA%26A..50..211K)

> **Comments:**
> **username_0**: Please see the update to the question... I don't see how most of the things mentioned here could migrate a large plant to be so close to the star, in what seems like a stable orbit.  
> **username_1**: I'm not sure to see the point of your comment. I gave you six possible mechanisms that can cause migration. And there is no such thing as a stable orbit in the long run, as soon as you have 3 bodies or more in a gravitational system, it is a chaotic system, and the objects can migrate inward or outward. So first thing is: we observe planets very close to their host star, that can be explained only by migration. Then, they could probably migrate outward, as it happened in our Solar System.  
> **username_0**: Thanks MBR. I like the information presented here, but I was looking for "accepted theories that successfully predicted" this before it was observed.  The 4 theories mentioned all try to explain it, but they did not require planets to be in such a close orbit. The only system we have reliable information about (so far) is our own, and as you say, the planets here are migrating outward.  
> **username_2**: there is also selection bias, our previous methods of planet detection, gained higher weight for close to the star planets  

---

### Answer by username_3 on 2013-10-23. Score: 6

Because more massive, closely-orbiting planet exert much greater gravitational forces on their host star than smaller, more distant planets, there is a *significant* observational bias. We are much more likely to detect these planets (i.e. "[hot Jupiters](http://en.wikipedia.org/wiki/Hot_Jupiter)") because their observable effects (Doppler wobble, gravitational lensing, etc) are more significant.

The below image ([from here](http://en.wikipedia.org/wiki/Exoplanet#Orbital_parameters)) shows you that we're still mostly unable to detect planets like those in our own Solar system (the gray circles are our Solar planets). It's just too hard.

I don't study planetary formation (yet!), so I can't speak well for the theoretical side of things. All we've managed to collect so far is the very lowest-hanging fruit, so it is highly likely that there are is a [large number](http://www.space.com/19659-alien-earth-exoplanets-red-dwarfs.html) of Earth-like planets out there too.

The authors of [this article](http://mnras.oxfordjournals.org/content/430/2/951) on the Kepler-10 system (you'll need MNRAS access) suggest planet-planet gravitational scattering or collision-merger events. The [Scholarpedia](http://www.scholarpedia.org/article/Planetary_formation_and_migration) article provides a bit more brief reading on these mechanisms. [Here is a review article](http://www.eos.ubc.ca/~mjelline/453website/eosc453/E_prints/newfer010/chambers_planetarymigration_AR09.pdf) from 2009, and a more [in-depth review](http://arxiv.org/abs/astro-ph/0510487) from 2006.

![Exoplanets discovered up until 2010-10-03.](https://i.sstatic.net/XWp3B.png)

> **Comments:**
> **username_1**: It does not really answer the question of the OP; you explain instead why we detect them. It is also interesting, but irrelevant here.  
> **username_0**: Thanks. This is interesting too, showing how many are very close!  
> **username_0**: Your link brought me to http://en.wikipedia.org/wiki/Planetary_migration which touches on this, but doesn't seem to mention any theory that predicted what we are seeing now.  
> **username_3**: In essence, the OP is wondering why so many planets are being found very close to their host stars. My answer explains *why* so many are *found* close to the star. MBR explains, much better than I could, *how* they get there. I think that both answers work together. @GlenLittle - see my edited answer. I have found some more references that outline current theory.  
> **username_4**: Yes, the problem is difference between "Are close to the star" and "Are observed" close to the star". There may be a vast number of planets distant from the stars and completely overshadowing the number that is close to the stars. We're just unable to detect them. It's like "Why are there so many more white cats out at night than black ones?" - no, the proportions are the same, you just don't see the black cats because they hide in darkness better.  

---

