<p>There are two primary reasons <a href="https://en.wikipedia.org/wiki/Planet_Nine" rel="noreferrer">Planet Nine</a> (if it exists) is extremely hard to detect: it's very dim and its position is not well constrained.</p>

<ul>
<li><p><strong>Brightness</strong> - We are able to see planets in our solar system because they reflect light from the sun.  The amount of light they reflect depends on how large the object is, how reflective it is, and (most importantly) the amount of sunlight that hits it.  The hypothesized distance at which Planet Nine is orbiting is so far away that it will receive only a tiny amount of sunlight. At this distance, its apparent magnitude would be greater than 22.  In comparison, <a href="https://en.wikipedia.org/wiki/Pluto" rel="noreferrer">Pluto's apparent magnitude</a> is about 15.  Magnitude is a logarithmic scale, so that means that Planet Nine would be $631$ times dimmer than Pluto, making it incredibly hard to see.</p></li>
<li><p><strong>Unknown Position</strong> - The estimates of Planet Nine's characteristics are based on how it gravitationally disrupts the orbits of some trans-Neptunian objects.  Based on these observations, scientists can calculate where it is likely to be, but these calculations depend on its size.  <a href="https://www.washingtonpost.com/news/speaking-of-science/wp/2016/01/20/new-evidence-suggests-a-ninth-planet-lurking-at-the-edge-of-the-solar-system/" rel="noreferrer">Estimates of its mass</a> range from 2 to 4 times the mass of the Earth.  And all of these are just estimates, giving us a relatively large patch of sky to search to find Planet Nine.</p></li>
</ul>

<p>Combining both of these reasons, it is very difficult to search for Planet Nine.  We're looking for an incredibly dim object in a large area of the sky.</p>

<hr>

<p>Now, when we look for exoplanets, things are actually a lot easier.  To <a href="https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanets" rel="noreferrer">detect exoplanets</a> we hardly ever look for the planets directly.  We've only detected about <a href="https://en.wikipedia.org/wiki/List_of_directly_imaged_exoplanets" rel="noreferrer">20 planets</a> via direct imaging.  Instead, we look at the light from the star.  The stars are many orders of magnitude brighter than the planets, and are very easy to see.  All we have to do is see how the light from the star changes over time.  An early method detect shifts in the radial velocity of the star (its motion towards or away from us) using <a href="https://en.wikipedia.org/wiki/Doppler_spectroscopy" rel="noreferrer">Doppler spectroscopy</a>.  The more prevalent method today, the one that the Kepler mission used, is <a href="https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanets#Transit_photometry" rel="noreferrer">transit photometry</a>.  We measure the light output of a star, and look for dips in its intensity, caused by the planet orbiting in front of the star and blocking some of the light.</p>

<p>Because we're looking at a star, we don't have the problems we have when looking for Planet Nine.  Stars are very bright, their positions are known very accurately, and we don't have to wait too long to make a detection.  Plus, surveys like <a href="https://www.nasa.gov/mission_pages/kepler/overview/index.html" rel="noreferrer">Kepler</a> are able to scan many, many stars at once.</p>

CC BY-SA 3.0
2017-02-25T19:28:15.177Z
Planet 9 is "moving" very rapidly in the sky. If it is found it will be recognised by its enormous parallax. If it is 10,000 au from the Sun, the parallax is 20 arcseconds and it will noticeably move in a couple of weeks. The comment about 7 magnitudes equalling $10^7$ is wildly incorrect also.
ProfRob
CC BY-SA 3.0
2017-02-25T19:59:22.277Z
@RobJeffries You're right about the magnitudes; I was thinking distance modulus for some reason.  And while it may have a large parallax, it is still a much smaller change than Pluto.  I'll edit my answer, though.
Phiteros
CC BY-SA 3.0
2017-02-25T18:38:50.197Z
2017-02-25T19:58:15.313Z
2017-02-25T19:58:15.313Z
Phiteros
<p>According to NASA, we've discovered 3,453 exoplanets so far<a href="http://exoplanetarchive.ipac.caltech.edu/" rel="noreferrer"> [1]</a>. With some of the farthest being at 13,000 light years away<a href="http://www.space.com/29120-alien-planet-among-farthest-known.html" rel="noreferrer"> [2]</a>. This would suggest today scientists have a really advanced capability for this kind of discoveries. However, the existence of a ninth planet in the solar system (which has not only been proposed but its characteristics are already predicted with great precision<a href="https://en.wikipedia.org/wiki/Planet_Nine" rel="noreferrer">[3]</a>) has not yet been discovered. If a ninth planet existed in our solar system, shouldn't it have already been discovered? How can exoplanets thousands of light years away be discovered weekly, while a giant planet right in our backyard still lays undiscovered?</p>

CC BY-SA 3.0
2017-02-25T18:05:00.587Z
The short answer: really, really long orbital period.
zibadawa timmy
CC BY-SA 3.0
2017-02-25T19:23:54.797Z
Answered in http://physics.stackexchange.com/questions/230973/how-can-we-see-planets-thousands-of-light-years-away-but-dont-know-if-there-are
ProfRob
CC BY-SA 3.0
2017-02-25T20:23:42.447Z
Also strongly related: http://astronomy.stackexchange.com/questions/8605/if-alpha-centauri-as-solar-system-exactly-mirrored-our-own-what-would-we-be-ab
ProfRob
CC BY-SA 3.0
2017-02-27T14:10:22.303Z
Imagine trying to find a dirty penny on the ground in your back yard at night while looking out from your bedroom window vs watching the streetlight down the street dim a bit. Just because the dirty penny is nearby, doesn't make it any easier to find than seeing a streetlight get dimmer.
zephyr
<planet><telescope><exoplanet><9th-planet>
If thousands of exoplanets have been discovered, why haven't we discovered planet 9 yet?
Gabe12
# If thousands of exoplanets have been discovered, why haven't we discovered planet 9 yet?

(Asked by: username_0 on 2017-02-25)

According to NASA, we've discovered 3,453 exoplanets so far [[1]](http://exoplanetarchive.ipac.caltech.edu/). With some of the farthest being at 13,000 light years away [[2]](http://www.space.com/29120-alien-planet-among-farthest-known.html). This would suggest today scientists have a really advanced capability for this kind of discoveries. However, the existence of a ninth planet in the solar system (which has not only been proposed but its characteristics are already predicted with great precision[[3]](https://en.wikipedia.org/wiki/Planet_Nine)) has not yet been discovered. If a ninth planet existed in our solar system, shouldn't it have already been discovered? How can exoplanets thousands of light years away be discovered weekly, while a giant planet right in our backyard still lays undiscovered?

**Tags:** planet, telescope, exoplanet, 9th-planet

> **Comments:**
> **username_1**: The short answer: really, really long orbital period.  
> **username_2**: Answered in http://physics.stackexchange.com/questions/230973/how-can-we-see-planets-thousands-of-light-years-away-but-dont-know-if-there-are  
> **username_2**: Also strongly related: http://astronomy.stackexchange.com/questions/8605/if-alpha-centauri-as-solar-system-exactly-mirrored-our-own-what-would-we-be-ab  
> **username_3**: Imagine trying to find a dirty penny on the ground in your back yard at night while looking out from your bedroom window vs watching the streetlight down the street dim a bit. Just because the dirty penny is nearby, doesn't make it any easier to find than seeing a streetlight get dimmer.  

---

## 1 answers

### Answer by username_4 on 2017-02-25. Score: 7 | Accepted answer

There are two primary reasons [Planet Nine](https://en.wikipedia.org/wiki/Planet_Nine) (if it exists) is extremely hard to detect: it's very dim and its position is not well constrained.

* **Brightness** - We are able to see planets in our solar system because they reflect light from the sun. The amount of light they reflect depends on how large the object is, how reflective it is, and (most importantly) the amount of sunlight that hits it. The hypothesized distance at which Planet Nine is orbiting is so far away that it will receive only a tiny amount of sunlight. At this distance, its apparent magnitude would be greater than 22. In comparison, [Pluto's apparent magnitude](https://en.wikipedia.org/wiki/Pluto) is about 15. Magnitude is a logarithmic scale, so that means that Planet Nine would be $631$ times dimmer than Pluto, making it incredibly hard to see.
* **Unknown Position** - The estimates of Planet Nine's characteristics are based on how it gravitationally disrupts the orbits of some trans-Neptunian objects. Based on these observations, scientists can calculate where it is likely to be, but these calculations depend on its size. [Estimates of its mass](https://www.washingtonpost.com/news/speaking-of-science/wp/2016/01/20/new-evidence-suggests-a-ninth-planet-lurking-at-the-edge-of-the-solar-system/) range from 2 to 4 times the mass of the Earth. And all of these are just estimates, giving us a relatively large patch of sky to search to find Planet Nine.

Combining both of these reasons, it is very difficult to search for Planet Nine. We're looking for an incredibly dim object in a large area of the sky.

---

Now, when we look for exoplanets, things are actually a lot easier. To [detect exoplanets](https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanets) we hardly ever look for the planets directly. We've only detected about [20 planets](https://en.wikipedia.org/wiki/List_of_directly_imaged_exoplanets) via direct imaging. Instead, we look at the light from the star. The stars are many orders of magnitude brighter than the planets, and are very easy to see. All we have to do is see how the light from the star changes over time. An early method detect shifts in the radial velocity of the star (its motion towards or away from us) using [Doppler spectroscopy](https://en.wikipedia.org/wiki/Doppler_spectroscopy). The more prevalent method today, the one that the Kepler mission used, is [transit photometry](https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanets#Transit_photometry). We measure the light output of a star, and look for dips in its intensity, caused by the planet orbiting in front of the star and blocking some of the light.

Because we're looking at a star, we don't have the problems we have when looking for Planet Nine. Stars are very bright, their positions are known very accurately, and we don't have to wait too long to make a detection. Plus, surveys like [Kepler](https://www.nasa.gov/mission_pages/kepler/overview/index.html) are able to scan many, many stars at once.

> **Comments:**
> **username_2**: Planet 9 is "moving" very rapidly in the sky. If it is found it will be recognised by its enormous parallax. If it is 10,000 au from the Sun, the parallax is 20 arcseconds and it will noticeably move in a couple of weeks. The comment about 7 magnitudes equalling $10^7$ is wildly incorrect also.  
> **username_4**: @RobJeffries You're right about the magnitudes; I was thinking distance modulus for some reason.  And while it may have a large parallax, it is still a much smaller change than Pluto.  I'll edit my answer, though.  

---

