# Identifying extra-solar planets **URL:** \ **Category:** Factual Questions\ **Created:** [June 19, 2015, 5:32pm UTC](https://boards.straightdope.com/t/identifying-extra-solar-planets/722957 "2015-06-19T17:32:14Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Kelevra](https://avatars.discourse-cdn.com/v4/letter/k/97f17d/32.png) [@Kelevra](https://boards.straightdope.com/u/Kelevra)\ **Post date:** [June 19, 2015, 5:32pm UTC](https://boards.straightdope.com/t/identifying-extra-solar-planets/722957/1 "2015-06-19T17:32:14Z")
I read [this article](http://www.cnn.com/2015/06/11/europe/intern-discovers-planet/) about a high school intern that discovered a planet. The story says that the planet is the size of Jupiter and orbits its star in 2 days! Do we actually have a model that suggests this is possible? The planet would have to be so close to its star that I would think the tidal forces would pull it apart. At any distance from the star its velocity would be enormous (I think Ludicrous Speed would be an appropriate description). For comparison Mercury’s orbital period is 88 days. Does anyone have a good link to how the science of these measurements works? I’m curious about the amount of error that is in these measurements. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [June 19, 2015, 6:01pm UTC](https://boards.straightdope.com/t/identifying-extra-solar-planets/722957/2 "2015-06-19T18:01:18Z")
[Exoplanet orbits](https://en.wikipedia.org/wiki/Exoplanet#Orbital_parameters) Because it is far easier to detect abnormalities from large planets near suns, a disproportional number of exoplanets fit this model. ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [June 19, 2015, 6:02pm UTC](https://boards.straightdope.com/t/identifying-extra-solar-planets/722957/3 "2015-06-19T18:02:28Z")
For a body being torn apart by tidal forces, try this article: > **[Roche limit](https://en.wikipedia.org/wiki/Roche_limit)** > > In celestial mechanics, the Roche limit, also called Roche radius, is the distance from a celestial body within which a second celestial body, held together only by its own force of gravity, will disintegrate because the first body's tidal forces exceed the second body's self-gravitation. Inside the Roche limit, orbiting material disperses and forms rings, whereas outside the limit, material tends to coalesce. The Roche radius depends on the radius of the first body and on the ratio of the bod Th... ---
**Author:** ![Kelevra](https://avatars.discourse-cdn.com/v4/letter/k/97f17d/32.png) [@Kelevra](https://boards.straightdope.com/u/Kelevra)\ **Post date:** [June 19, 2015, 6:21pm UTC](https://boards.straightdope.com/t/identifying-extra-solar-planets/722957/4 "2015-06-19T18:21:19Z")
Thanks for the links. I guess it is counter intuitive (to me at least) but looking at the equations, the larger the mass of the planet the closer it could be to its primary. Its greater mass keeps it from breaking up. I find astronomy amazing. The size of objects and distances involved make me feel like my head is stretching just to understand it. ---
**Author:** ![gnoitall](https://avatars.discourse-cdn.com/v4/letter/g/bb73d2/32.png) [@gnoitall](https://boards.straightdope.com/u/gnoitall)\ **Post date:** [June 19, 2015, 6:27pm UTC](https://boards.straightdope.com/t/identifying-extra-solar-planets/722957/5 "2015-06-19T18:27:43Z")
> [@Kelevra](#): > > I read [this article](http://www.cnn.com/2015/06/11/europe/intern-discovers-planet/) about a high school intern that discovered a planet. The story says that the planet is the size of Jupiter and orbits its star in 2 days! > > Do we actually have a model that suggests this is possible? > > The planet would have to be so close to its star that I would think the tidal forces would pull it apart. At any distance from the star its velocity would be enormous (I think Ludicrous Speed would be an appropriate description). For comparison Mercury’s orbital period is 88 days. > > Does anyone have a good link to how the science of these measurements works? I’m curious about the amount of error that is in these measurements. Assume a solar mass, solar radius star. Assume a rigid Jupiter-mass planet. The rigid-body Roche limit for a Jupiter-mass planet orbiting a Sun-like star is [894,000 km.](https://en.wikipedia.org/wiki/Roche_limit) An orbital radius with a two-day revolution period[is .032 au](http://hyperphysics.phy-astr.gsu.edu/hbase/solorb.html), or 4,787,131 km. The planet is safe. It is travelling crazy fast, and it’s probably hotter than hell, but it’s not in imminent danger of breaking up. More to your original point, the measurements cited are credible from a tidal force perspective. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 19, 2015, 8:51pm UTC](https://boards.straightdope.com/t/identifying-extra-solar-planets/722957/6 "2015-06-19T20:51:33Z")
Actually, it’s not the mass of the planet that’s relevant, but its density (and also its shape, but realistic planets are all going to be about the same shape). At any given density, if you increase the mass, you’ll increase the force holding the planet together (its own gravity), but you’ll also increase the force that’s trying to pull it apart (tidal forces depend on the size of the object).