<p>The possible planet 9 is thought to be about 10 Earth masses and is unlikely to be a gas giant (it may be the core of an "interrupted" gas giant). As such, it will not be generating significant luminosity itself and would be rocky, or more likely, icy in character. It would thus only be seen by reflected light.</p>

<p>The considerations for what wavelength to search in balance the sensitivity of the instruments at hand with the likely spectrum of the object. This in turn depends on the solar spectrum and the wavelength dependence of the reflectivity (albedo).</p>

<p>For most icy objects, including Pluto and Trans-Neptunian objects, the reflectance increases to the red and near-infrared, whilst the solar spectrum peaks at shorter wavelengths. This suggests that searches are best carried out with wide field optical instruments in the R or r' bands at around 600 nm.</p>

<p>A further factor in <em>finding</em> a candidate is that you are going to have to cover a large area. This is only feasible at optical and NIR wavelengths unless the object was bright enough in the mid-IR to show up in WISE (which I'm sure is being thoroughly checked). A press release I saw said SUBARU is being used for the search. I would bet they are using the half degree field of Suprime-Cam at optical wavelengths and not pursuing COMICS mid-IR imaging with it 42x32 <em>arcsecond</em> field!</p>

<p>Confirming a candidate should be easy, given the enormous parallax and proper motion expected.</p>

CC BY-SA 3.0
2016-01-22T14:22:58.063Z
I'm not sure that kbo observations would be valid. This object is predicted to be much larger that a typical kbo...
PearsonArtPhoto
CC BY-SA 3.0
2016-01-22T14:24:42.617Z
@PearsonArtPhoto Feel free to make up your own reflectance spectrum. Even if it were flat that might only shift the search range to the V or g' bands.
ProfRob
CC BY-SA 3.0
2016-01-22T15:10:42.457Z
The reflectance spectrum is probably close, I will give you that, but I'm somewhat more skeptical of the thermal IR detection possibilities. See my answer.
PearsonArtPhoto
CC BY-SA 3.0
2016-01-22T14:08:00.293Z
2016-01-22T16:23:03.473Z
2016-01-22T16:23:03.473Z
ProfRob
<p>There are two basic ways to detect such an object. First is to detect it through reflected sunlight. Second is from the heat that it produces. We already know that the <a href="https://astronomy.stackexchange.com/a/13275/182">reflected light of such an object</a> likely would be around a 16.5 magnitude. To determine the infrared, we have to estimate the temperature</p>

<p>The temperature very much depends on the composition. For simplicity, let's assume a composition similar to Earth, and was created about the same time as the rest of the Solar System. These assumptions may not prove to be valid, but they are among the possibilities discussed. Earth's internal heat, in fact, is at least 50% from radioactive decay, according to <a href="http://blogs.scientificamerican.com/observations/nuclear-fission-confirmed-as-source-of-more-than-half-of-earths-heat/" rel="nofollow noreferrer">Scientific America</a>. Of course, that's the internal heat only, not all of that will make it to the surface. </p>

<p>This proposed planet is somewhat akin to a <a href="https://en.wikipedia.org/wiki/Rogue_planet#Retention_of_heat_in_interstellar_space" rel="nofollow noreferrer">"Rogue Planet"</a>, where a small disk of gas collapsed into a planet without a star, or were ejected from their host system. A fair bit also depends on if there is a sizable moon of the object. If so, then tidal heating would dramatically increase the temperature of the object. Any such determination can't be made without observation, but it is possible. An atmosphere would also help to keep the planet from freezing. A <a href="http://arxiv.org/pdf/1102.1108v2.pdf" rel="nofollow noreferrer">paper for detecting rogue planets</a> comes from Abbott and Switzer. They hypothesis that a 3.5 Earth Mass object could be detected if it comes within 1000 AU, specifically in the far infrared, with a surface temperature of about 50 K.</p>

<p>Bottom line, it would probably be wise to try to detect both in the far infrared, as well as the visible, although it might be difficult to detect, even then. Given parallax as the primary means of motion, the detection should be done at several points in Earth's orbit, probably the same spot should be searched about 90 days apart to give the maximum opportunity to move, as parallax would only be visible if the motion of the Earth was perpendicular to the location of the object.</p>

CC BY-SA 3.0
2016-01-22T15:06:17.263Z
A third method would be to discover several more comets (or TNOs or KBOs) who have had their orbits shaped by this Ninth guy. Even if directly unobserved, a planet like this will become more real if the evidence stacks up. And IR survey space telescopes, and maybe other survey telescopes, in the pipeline should improve the detectability of its "victims" by alot.
LocalFluff
CC BY-SA 3.0
2016-01-22T15:09:43.670Z
True enough, but no one is really going to consider it solved until they have detection of it. All that further evidence really does is to shrink the part of the sky to look for it.
PearsonArtPhoto
CC BY-SA 3.0
2016-01-22T16:19:47.750Z
I do not think there are any far-IR survey-type facilities and this has to be factored in. Sure, look for it in the far-IR *after* it has been found. The search is taking place with Subaru according to the press release. My guess is they are using the half degree field of the optical SuprimeCam and not the 42x32 arcsec of the COMICS near and mid-IR camera !
ProfRob
CC BY-SA 3.0
2016-01-23T11:20:44.490Z
I'm not sure how realistic this approach is, but in theory wouldn't you be able to find Planet9 by looking for objects that are hidden? By that I mean, make comparisons of the sky and find objects that are missing that normally should be visible.
TryHarder
CC BY-SA 3.0
2016-01-22T14:58:07.957Z
2016-01-22T16:05:42.280Z
2017-04-13T12:59:06.910Z
PearsonArtPhoto
<p>Direct reflection of sunlight is the most likely scenario for a ninth planet discovery, however that does not hold if the object has a very low albedo. I assume you are interested in what wavelengths the planet would radiate.</p>

<p>For the surface temperature, the rotation of the planet is important. If it is locked with one side facing the sun, or rotates very slowly, the centre of the sun facing hemisphere radiates as much energy as it gets from the Sun. At 60 AU, the solar flux is about 0.38 W/m². Using the <a href="https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law">Stefan-Boltzmann law</a>, we obtain a equilibrium surface temperature of 51 K (that is the highest possible surface temperature, assuming it does not have an atmosphere).
<a href="https://en.wikipedia.org/wiki/Wien&#39;s_displacement_law">Wien's displacement law</a> tells us that radiation from a 51 k object peaks at a wavelength of 57 µm (infra-red).</p>

<p>For a rotating body, the equator temperature is 38 K, with radiation peaking at 78 µm (still infra-red).</p>

<p>Using an albedo of 0.5, the peaks are 68 µm and 90 µm for a non-rotating and a rotating body respectively.
Note that this is for the equator region only, the actual peak-wavelength is going to be a little bit higher, belonging in the far infra-red spectrum. Also, the high uncertainty of rotation, albedo and mass (mass is important for internal heat), makes it impossible to get a higher accuracy than that</p>

<p>60 au is a very optimistic perihelion distance for the ninth planet, so for a more realistic distance of say 200 au, it is not possible to observe it in the IR spectrum, if it does not have a significant internal heat source.</p>

CC BY-SA 3.0
2016-01-22T14:59:10.650Z
Interesting, although I believe radioactive decay, among other factors, would also be a significant influencer in the surface heat.
PearsonArtPhoto
CC BY-SA 3.0
2016-01-22T16:14:41.553Z
You have to factor in how difficult it is to measure anything at those wavelengths. You would need a working cryogenic IR space telescope for one thing.
ProfRob
CC BY-SA 3.0
2016-01-22T16:27:37.680Z
The possible planet 9 is a lot further away than 60 au ? It will only be detectable in the IR if it has some sort of internal heat generation.
ProfRob
CC BY-SA 3.0
2016-01-23T10:01:39.210Z
60 au is best case
SE - stop firing the good guys
CC BY-SA 3.0
2016-01-24T22:03:41.467Z
I have to say, I have yet to hear of 60 AU being used, even as a perihelion distance. The figures I've seen are easily in the hundreds of AU. I'm curious to know your source for that. This is just a minor point, though - maybe.
HDE 226868
CC BY-SA 3.0
2016-01-24T22:07:35.200Z
@HDE226868 I picked 60 AU because that is how far away from the sun you can observe it in the IR spectrum. I am aware that the estimates are far larger, 60 AU is just a limit. (this says for instance 200 AU http://www.findplanetnine.com/p/blog-page.html)
SE - stop firing the good guys
CC BY-SA 3.0
2016-01-24T22:09:46.513Z
Ah, I understand.
HDE 226868
CC BY-SA 3.0
2016-01-24T23:05:19.590Z
It will spend a tiny fraction of its orbit near perihelion. Most likely distance would be more like 1000 au.
ProfRob
CC BY-SA 3.0
2016-01-22T14:55:47.000Z
2016-01-24T22:01:00.877Z
2016-01-24T22:01:00.877Z
SE - stop firing the good guys
<p>We know that the reflected sunlight will make detecting the 9th planet very difficult in the visible light. Is there another band that will be more likely to detect it? What is the surface temperature of this object likely to be, and what would that mean about its optimal detection wavelength?</p>

CC BY-SA 3.0
2016-01-22T12:16:52.010Z
**If it exists**.
David Hammen
CC BY-SA 3.0
2016-01-22T12:17:45.063Z
Even if it doesn't exist, asking a question like this helps to decide if in fact it does exist.
PearsonArtPhoto
CC BY-SA 3.0
2016-01-22T12:38:44.397Z
The WISE all-sky survey was supposed to have put the issue of Planet-X to bed. As is the case with the supposedly super-luminal neutrinos, I think the best thing to do with this new supposed Planet-X is to give it a rest. If this planet exists, the WISE all-sky survey should have seen it.
David Hammen
CC BY-SA 3.0
2016-01-22T12:39:59.377Z
That's really a separate question, I would suggest that you ask why WISE didn't detect this object, which could lead to some interesting discussion.
PearsonArtPhoto
CC BY-SA 3.0
2016-01-22T12:41:53.563Z
That is, unless it's a planet that's more like Uranus than Neptune. Uranus is a bit of an outlier compared to the other giant planets. Jupiter, Saturn, and Neptune emit over 2.5 times the radiation they receive from the Sun. The Kelvin-Helmhotz timescale dominates for these three planets. Uranus OTOH emits a mere 10% more radiation than it receives from the Sun.
David Hammen
CC BY-SA 3.0
2016-01-22T12:44:11.477Z
All bets are off if this supposed ninth planet is a magically captured rogue planet that is much older than the solar system. In that case, it will have cooled to the state of near invisibility.
David Hammen
CC BY-SA 3.0
2016-01-22T13:51:28.760Z
@DavidHammen This planet 9 is likely not a gas giant. It is only about 10 Earth masses and probably rocky or icy. It will not be bright in the infrared and it would not necessarily have been detected by WISE at all.
ProfRob
CC BY-SA 3.0
2016-01-22T14:09:37.400Z
@RobJeffries -- Possibly. As planned, the WISE all-sky surgery was supposedly going to detect Jupiter-sized objects out to multiple tens of thousands of AUs, and Neptune-size objects out to a thousand AU or so. That's assuming a lot, that the object in question formed at the same time as the solar system, and also assuming a lot about the object's luminosity. (Last time I checked, the lower than expected thermal radiation from Uranus is still highly problematic.) If those assumptions are valid, there is no Planet X with a mass of ~10 Earth masses in the range posited by Brown et. al.
David Hammen
CC BY-SA 3.0
2016-01-22T16:54:09.717Z
@DavidHammen Yes, there is no "Saturn" within 10,000 au. But this possible planet 9 is probably not a gas giant. Unless it were significantly heated by radioactivity or tides in some way (see answers below), it could not possibly be seen by WISE at 1000 au (its equilibrium temperature would be too low). Perhaps this is the reason (I have not seen any paper yet), why the authors say it is likely not a gas giant. Do you have a reference/URL for where a calculation for a Neptune-sized object in WISE is done? I can only find limits for Jupiters/Saturns.
ProfRob
CC BY-SA 3.0
2016-01-22T18:59:18.993Z
The initially advertised capabilities of the all-sky survey was to detect whether a Jupiter-sized object existed within a few light years of the solar system, and a Neptune-sized body within several hundred AU.
David Hammen
CC BY-SA 3.0
2016-01-23T04:20:52.093Z
@DavidHammen - What would a simple extrapolation say about a Planet X sized object? And how far away do they claim #9 is?
Mazura
<solar-system><planet><observational-astronomy><9th-planet><wavelength>
What wavelength to best detect the "9th planet"?
PearsonArtPhoto
# What wavelength to best detect the "9th planet"?

(Asked by: username_0 on 2016-01-22)

We know that the reflected sunlight will make detecting the 9th planet very difficult in the visible light. Is there another band that will be more likely to detect it? What is the surface temperature of this object likely to be, and what would that mean about its optimal detection wavelength?

**Tags:** solar-system, planet, observational-astronomy, 9th-planet, wavelength

> **Comments:**
> **username_1**: **If it exists**.  
> **username_0**: Even if it doesn't exist, asking a question like this helps to decide if in fact it does exist.  
> **username_1**: The WISE all-sky survey was supposed to have put the issue of Planet-X to bed. As is the case with the supposedly super-luminal neutrinos, I think the best thing to do with this new supposed Planet-X is to give it a rest. If this planet exists, the WISE all-sky survey should have seen it.  
> **username_0**: That's really a separate question, I would suggest that you ask why WISE didn't detect this object, which could lead to some interesting discussion.  
> **username_1**: That is, unless it's a planet that's more like Uranus than Neptune. Uranus is a bit of an outlier compared to the other giant planets. Jupiter, Saturn, and Neptune emit over 2.5 times the radiation they receive from the Sun. The Kelvin-Helmhotz timescale dominates for these three planets. Uranus OTOH emits a mere 10% more radiation than it receives from the Sun.  
> **username_1**: All bets are off if this supposed ninth planet is a magically captured rogue planet that is much older than the solar system. In that case, it will have cooled to the state of near invisibility.  
> **username_2**: @DavidHammen This planet 9 is likely not a gas giant. It is only about 10 Earth masses and probably rocky or icy. It will not be bright in the infrared and it would not necessarily have been detected by WISE at all.  
> **username_1**: @RobJeffries -- Possibly. As planned, the WISE all-sky surgery was supposedly going to detect Jupiter-sized objects out to multiple tens of thousands of AUs, and Neptune-size objects out to a thousand AU or so. That's assuming a lot, that the object in question formed at the same time as the solar system, and also assuming a lot about the object's luminosity. (Last time I checked, the lower than expected thermal radiation from Uranus is still highly problematic.) If those assumptions are valid, there is no Planet X with a mass of ~10 Earth masses in the range posited by Brown et. al.  
> **username_2**: @DavidHammen Yes, there is no "Saturn" within 10,000 au. But this possible planet 9 is probably not a gas giant. Unless it were significantly heated by radioactivity or tides in some way (see answers below), it could not possibly be seen by WISE at 1000 au (its equilibrium temperature would be too low). Perhaps this is the reason (I have not seen any paper yet), why the authors say it is likely not a gas giant. Do you have a reference/URL for where a calculation for a Neptune-sized object in WISE is done? I can only find limits for Jupiters/Saturns.  
> **username_1**: The initially advertised capabilities of the all-sky survey was to detect whether a Jupiter-sized object existed within a few light years of the solar system, and a Neptune-sized body within several hundred AU.  
> **username_3**: @DavidHammen - What would a simple extrapolation say about a Planet X sized object? And how far away do they claim #9 is?  

---

## 3 answers

### Answer by username_6 on 2016-01-22. Score: 7 | Accepted answer

Direct reflection of sunlight is the most likely scenario for a ninth planet discovery, however that does not hold if the object has a very low albedo. I assume you are interested in what wavelengths the planet would radiate.

For the surface temperature, the rotation of the planet is important. If it is locked with one side facing the sun, or rotates very slowly, the centre of the sun facing hemisphere radiates as much energy as it gets from the Sun. At 60 AU, the solar flux is about 0.38 W/m². Using the [Stefan-Boltzmann law](https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law), we obtain a equilibrium surface temperature of 51 K (that is the highest possible surface temperature, assuming it does not have an atmosphere).
[Wien's displacement law](https://en.wikipedia.org/wiki/Wien's_displacement_law) tells us that radiation from a 51 k object peaks at a wavelength of 57 µm (infra-red).

For a rotating body, the equator temperature is 38 K, with radiation peaking at 78 µm (still infra-red).

Using an albedo of 0.5, the peaks are 68 µm and 90 µm for a non-rotating and a rotating body respectively.
Note that this is for the equator region only, the actual peak-wavelength is going to be a little bit higher, belonging in the far infra-red spectrum. Also, the high uncertainty of rotation, albedo and mass (mass is important for internal heat), makes it impossible to get a higher accuracy than that

60 au is a very optimistic perihelion distance for the ninth planet, so for a more realistic distance of say 200 au, it is not possible to observe it in the IR spectrum, if it does not have a significant internal heat source.

> **Comments:**
> **username_0**: Interesting, although I believe radioactive decay, among other factors, would also be a significant influencer in the surface heat.  
> **username_2**: You have to factor in how difficult it is to measure anything at those wavelengths. You would need a working cryogenic IR space telescope for one thing.  
> **username_2**: The possible planet 9 is a lot further away than 60 au ? It will only be detectable in the IR if it has some sort of internal heat generation.  
> **username_6**: 60 au is best case  
> **username_7**: I have to say, I have yet to hear of 60 AU being used, even as a perihelion distance. The figures I've seen are easily in the hundreds of AU. I'm curious to know your source for that. This is just a minor point, though - maybe.  
> **username_6**: @HDE226868 I picked 60 AU because that is how far away from the sun you can observe it in the IR spectrum. I am aware that the estimates are far larger, 60 AU is just a limit. (this says for instance 200 AU http://www.findplanetnine.com/p/blog-page.html)  
> **username_7**: Ah, I understand.  
> **username_2**: It will spend a tiny fraction of its orbit near perihelion. Most likely distance would be more like 1000 au.  

---

### Answer by username_2 on 2016-01-22. Score: 9

The possible planet 9 is thought to be about 10 Earth masses and is unlikely to be a gas giant (it may be the core of an "interrupted" gas giant). As such, it will not be generating significant luminosity itself and would be rocky, or more likely, icy in character. It would thus only be seen by reflected light.

The considerations for what wavelength to search in balance the sensitivity of the instruments at hand with the likely spectrum of the object. This in turn depends on the solar spectrum and the wavelength dependence of the reflectivity (albedo).

For most icy objects, including Pluto and Trans-Neptunian objects, the reflectance increases to the red and near-infrared, whilst the solar spectrum peaks at shorter wavelengths. This suggests that searches are best carried out with wide field optical instruments in the R or r' bands at around 600 nm.

A further factor in *finding* a candidate is that you are going to have to cover a large area. This is only feasible at optical and NIR wavelengths unless the object was bright enough in the mid-IR to show up in WISE (which I'm sure is being thoroughly checked). A press release I saw said SUBARU is being used for the search. I would bet they are using the half degree field of Suprime-Cam at optical wavelengths and not pursuing COMICS mid-IR imaging with it 42x32 *arcsecond* field!

Confirming a candidate should be easy, given the enormous parallax and proper motion expected.

> **Comments:**
> **username_0**: I'm not sure that kbo observations would be valid. This object is predicted to be much larger that a typical kbo...  
> **username_2**: @username_0 Feel free to make up your own reflectance spectrum. Even if it were flat that might only shift the search range to the V or g' bands.  
> **username_0**: The reflectance spectrum is probably close, I will give you that, but I'm somewhat more skeptical of the thermal IR detection possibilities. See my answer.  

---

### Answer by username_0 on 2016-01-22. Score: 6

There are two basic ways to detect such an object. First is to detect it through reflected sunlight. Second is from the heat that it produces. We already know that the [reflected light of such an object](https://astronomy.stackexchange.com/a/13275/182) likely would be around a 16.5 magnitude. To determine the infrared, we have to estimate the temperature

The temperature very much depends on the composition. For simplicity, let's assume a composition similar to Earth, and was created about the same time as the rest of the Solar System. These assumptions may not prove to be valid, but they are among the possibilities discussed. Earth's internal heat, in fact, is at least 50% from radioactive decay, according to [Scientific America](http://blogs.scientificamerican.com/observations/nuclear-fission-confirmed-as-source-of-more-than-half-of-earths-heat/). Of course, that's the internal heat only, not all of that will make it to the surface.

This proposed planet is somewhat akin to a ["Rogue Planet"](https://en.wikipedia.org/wiki/Rogue_planet#Retention_of_heat_in_interstellar_space), where a small disk of gas collapsed into a planet without a star, or were ejected from their host system. A fair bit also depends on if there is a sizable moon of the object. If so, then tidal heating would dramatically increase the temperature of the object. Any such determination can't be made without observation, but it is possible. An atmosphere would also help to keep the planet from freezing. A [paper for detecting rogue planets](http://arxiv.org/pdf/1102.1108v2.pdf) comes from Abbott and Switzer. They hypothesis that a 3.5 Earth Mass object could be detected if it comes within 1000 AU, specifically in the far infrared, with a surface temperature of about 50 K.

Bottom line, it would probably be wise to try to detect both in the far infrared, as well as the visible, although it might be difficult to detect, even then. Given parallax as the primary means of motion, the detection should be done at several points in Earth's orbit, probably the same spot should be searched about 90 days apart to give the maximum opportunity to move, as parallax would only be visible if the motion of the Earth was perpendicular to the location of the object.

> **Comments:**
> **username_4**: A third method would be to discover several more comets (or TNOs or KBOs) who have had their orbits shaped by this Ninth guy. Even if directly unobserved, a planet like this will become more real if the evidence stacks up. And IR survey space telescopes, and maybe other survey telescopes, in the pipeline should improve the detectability of its "victims" by alot.  
> **username_0**: True enough, but no one is really going to consider it solved until they have detection of it. All that further evidence really does is to shrink the part of the sky to look for it.  
> **username_2**: I do not think there are any far-IR survey-type facilities and this has to be factored in. Sure, look for it in the far-IR *after* it has been found. The search is taking place with Subaru according to the press release. My guess is they are using the half degree field of the optical SuprimeCam and not the 42x32 arcsec of the COMICS near and mid-IR camera !  
> **username_5**: I'm not sure how realistic this approach is, but in theory wouldn't you be able to find Planet9 by looking for objects that are hidden? By that I mean, make comparisons of the sky and find objects that are missing that normally should be visible.  

---

