<p>The infrared satellite WISE did an all sky survey at near and mid-infrared wavelengths.</p>
<p>A nearby &quot;rogue planet&quot; would likely be moving quickly on the sky and if the size of Jupiter would emit substantial infrared radiation.</p>
<p>Close examination of the WISE survey data (in the context of looking for planet X) concluded there was unlikely to be Jupiter-sized planet <a href="https://www.nasa.gov/jpl/wise/pia17990/#.YxOWDlMo80N" rel="noreferrer">within 26,000 au</a>.</p>

CC BY-SA 4.0
2022-09-03T17:54:20.443Z
2022-09-03T18:00:30.747Z
2022-09-03T18:00:30.747Z
ProfRob
<h2>Rogue Planets are Discovered Mainly Through Micro-Lensing</h2>
<p>We have alreqdy discovered a number of rogue exoplanets, but we don't do it by directly imaging them. Rather, we detect them by watching their gravity distort the light of a star they pass across.</p>
<p>Data from the Kepler mission has been used to identify 27 rogue planet candidates, including four that may be earth-sized:</p>
<p><a href="https://academic.oup.com/mnras/article/505/4/5584/6315707?login=false" rel="nofollow noreferrer">Candidate short-duration events from the first space-based survey for planetary microlensing</a></p>
<p>The upcoming Nancy Roman Grace Telescope will survey the sky in the direction of the galactic bulge looking for micro-lensing events. It will use a wide-field camera to look at the same region of the galaxy repeatedly for transient events. It will be able to discover rogue planets down to the size of Mars.</p>
<p>I don't think the distance to us really matters much - it's more about the geometry between the rogue planet and the star it is lensing.  This technique will also be used to detect Earth-sized exoplanets around other stars.</p>
<p><a href="https://roman.gsfc.nasa.gov/galactic_bulge_time_domain_survey.html" rel="nofollow noreferrer">Nancy Roman Grace Galactic Bulge Time Domain Survey</a></p>

CC BY-SA 4.0
2022-09-02T22:18:13.783Z
Good info. - I wasn't aware of this work. I suppose the deflections are far too small to measure.  I would be nice to have a deflection curve as well as a light curve.
Roger Wood
CC BY-SA 4.0
2022-09-04T14:17:19.263Z
How does one make the difference between a rogue planet and a regular exoplanet transit? Especially in the case of an exoplanet with a very long orbit where we wouldn’t detect regularly repeating transits?
jcaron
CC BY-SA 4.0
2022-09-02T17:35:40.873Z
2022-09-03T21:54:23.003Z
2022-09-03T21:54:23.003Z
Dan Hanson
<p>All sky surveys tend to be at optical or radio wavelengths and more rarely at infra-red or x-ray wavelengths using space-based telescopes. The exposure times are short, so the chances of finding a cold dark quiet object in deep space are very slim. You might get very lucky a catch an occultation.</p>
<p>However, <em>assuming you know exactly where to look ...</em></p>
<p>Hubble can detect objects down to <a href="https://en.wikipedia.org/wiki/Limiting_magnitude" rel="noreferrer">about magnitude +30 albeit</a> with very long observation times. Jupiter shines in reflected light from the Sun at magnitude -2.7. Since reflected brightness falls as the 4th power of distance, its magnitude will drop as 2.5.log10(distance^4), so Jupiter would not be visible if it were more than 10^(32.7/2.5/4) = 1860 times further away. That would put it at a distance of about <em><strong>10,000 AU</strong></em>.  That distance would vary with the square root of the object's diameter and the fourth root of its albedo.</p>
<p>Jupiter also emits in the infra-red. Some that energy comes from the Sun, but most of it comes from inside Jupiter.  Assuming a temperature of 150K for a wandering Jupiter, that corresponds to a wavelength of 20 microns. This is in the range that JWST can sense (0.6 - 28.3 microns) though it is much less sensitive than at optical wavelengths. With many hours of observation, it might get down to <a href="https://www.stsci.edu/jwst/about-jwst/history/historical-sensitivity-estimates" rel="noreferrer">magnitude 23</a>. Assuming black body radiation, the Sun provides a good reference point at 6000K, 1.4E6 km diameter, 1AU with magnitude -27. So we could reduce its brightness by 50 magnitudes before it disappears.  That is a factor of 10^(50/2.5) = 10^20 in absolute brightness. Brightness goes as the fourth power of temperature, the square of diameter and the inverse square of distance. So a Jupiter sized object at 150K would become invisible to JWST at sqrt(10^20 * (150/6000)^4 * (143,000/1.4E6)^2) = <em><strong>640,000 AU</strong></em>. Remarkably, that's more than twice as far away as the nearest star.  But again, you would have to know exactly where to look and spend many many hours looking.</p>

CC BY-SA 4.0
2022-09-02T07:48:46.133Z
Thanks a bunch. Sounds as if there could be many such closer to us than Alpha Centauri, and we none the wiser. Perhaps some writer of juvenile sf will use such a place (or rather its moons) as a space pirates' hideout.
Mike Stone
CC BY-SA 4.0
2022-09-03T17:46:27.543Z
None the WISEr, how amusing.
ProfRob
CC BY-SA 4.0
2022-09-01T22:42:03.710Z
2022-09-01T22:42:03.710Z
Roger Wood
<p>I understand that there may be large numbers of &quot;orphan&quot; planets not circling any star.</p>
<p>If there were such a planet, say the size of Jupiter, in the vicinity of the Sun, about how close would it have to be in order for  telescopes to detect it?</p>

CC BY-SA 4.0
2022-08-26T12:00:13.900Z
I wuld have thought way out beyond the orbit of Pluto...
Slarty
CC BY-SA 4.0
2022-08-26T16:09:24.030Z
I took that as implied. I meant, are we talking light-years, or only hundreds or thousands of AUs?
Mike Stone
CC BY-SA 4.0
2022-08-27T07:56:00.980Z
I afraid I don't know and I suspect it is will be difficult to get a definitive answer because the likelihood of detection would just get greater and greater as the distance decreased. Although I would have thought someone here should be able to put an order of magnitude on it for you. The number will also continually evolve with time.
Slarty
CC BY-SA 4.0
2022-08-29T10:03:43.007Z
Wouldn't an orphan planet be 'dark'?  So in order to detect it, one'd have to detect its gravitational affect on bodies around it?   If no bodies are around them, then perhaps in the future, using gravitational waves as emitted by the planet?  Sorry... just speculating..
ewokx
CC BY-SA 4.0
2022-09-02T10:20:11.870Z
In ideal case than an orphan planet is young and very heavy (close to brown dwarf region, about 13 Jupiter's mass) - many light years, I suppose, msybe hundreds. Massive young orphan planet will be hot and cooling rather slowly. And it will be "size of Jupiter".
Heopps
CC BY-SA 4.0
2022-09-04T14:18:07.427Z
The usual name such a planet is a “rogue” planet.
jcaron
<observational-astronomy><rogue-planet>
Would a nearby "Orphan planet" be detectable?
Mike Stone
# Would a nearby "Orphan planet" be detectable?

(Asked by: username_0 on 2022-08-26)

I understand that there may be large numbers of "orphan" planets not circling any star.

If there were such a planet, say the size of Jupiter, in the vicinity of the Sun, about how close would it have to be in order for telescopes to detect it?

**Tags:** observational-astronomy, rogue-planet

> **Comments:**
> **username_1**: I wuld have thought way out beyond the orbit of Pluto...  
> **username_0**: I took that as implied. I meant, are we talking light-years, or only hundreds or thousands of AUs?  
> **username_1**: I afraid I don't know and I suspect it is will be difficult to get a definitive answer because the likelihood of detection would just get greater and greater as the distance decreased. Although I would have thought someone here should be able to put an order of magnitude on it for you. The number will also continually evolve with time.  
> **username_2**: Wouldn't an orphan planet be 'dark'?  So in order to detect it, one'd have to detect its gravitational affect on bodies around it?   If no bodies are around them, then perhaps in the future, using gravitational waves as emitted by the planet?  Sorry... just speculating..  
> **username_3**: In ideal case than an orphan planet is young and very heavy (close to brown dwarf region, about 13 Jupiter's mass) - many light years, I suppose, msybe hundreds. Massive young orphan planet will be hot and cooling rather slowly. And it will be "size of Jupiter".  
> **username_4**: The usual name such a planet is a “rogue” planet.  

---

## 3 answers

### Answer by username_7 on 2022-09-01. Score: 5 | Accepted answer

All sky surveys tend to be at optical or radio wavelengths and more rarely at infra-red or x-ray wavelengths using space-based telescopes. The exposure times are short, so the chances of finding a cold dark quiet object in deep space are very slim. You might get very lucky a catch an occultation.

However, *assuming you know exactly where to look ...*

Hubble can detect objects down to [about magnitude +30 albeit](https://en.wikipedia.org/wiki/Limiting_magnitude) with very long observation times. Jupiter shines in reflected light from the Sun at magnitude -2.7. Since reflected brightness falls as the 4th power of distance, its magnitude will drop as 2.5.log10(distance^4), so Jupiter would not be visible if it were more than 10^(32.7/2.5/4) = 1860 times further away. That would put it at a distance of about ***10,000 AU***. That distance would vary with the square root of the object's diameter and the fourth root of its albedo.

Jupiter also emits in the infra-red. Some that energy comes from the Sun, but most of it comes from inside Jupiter. Assuming a temperature of 150K for a wandering Jupiter, that corresponds to a wavelength of 20 microns. This is in the range that JWST can sense (0.6 - 28.3 microns) though it is much less sensitive than at optical wavelengths. With many hours of observation, it might get down to [magnitude 23](https://www.stsci.edu/jwst/about-jwst/history/historical-sensitivity-estimates). Assuming black body radiation, the Sun provides a good reference point at 6000K, 1.4E6 km diameter, 1AU with magnitude -27. So we could reduce its brightness by 50 magnitudes before it disappears. That is a factor of 10^(50/2.5) = 10^20 in absolute brightness. Brightness goes as the fourth power of temperature, the square of diameter and the inverse square of distance. So a Jupiter sized object at 150K would become invisible to JWST at sqrt(10^20 \* (150/6000)^4 \* (143,000/1.4E6)^2) = ***640,000 AU***. Remarkably, that's more than twice as far away as the nearest star. But again, you would have to know exactly where to look and spend many many hours looking.

> **Comments:**
> **username_0**: Thanks a bunch. Sounds as if there could be many such closer to us than Alpha Centauri, and we none the wiser. Perhaps some writer of juvenile sf will use such a place (or rather its moons) as a space pirates' hideout.  
> **username_5**: None the WISEr, how amusing.  

---

### Answer by username_5 on 2022-09-03. Score: 6

The infrared satellite WISE did an all sky survey at near and mid-infrared wavelengths.

A nearby "rogue planet" would likely be moving quickly on the sky and if the size of Jupiter would emit substantial infrared radiation.

Close examination of the WISE survey data (in the context of looking for planet X) concluded there was unlikely to be Jupiter-sized planet [within 26,000 au](https://www.nasa.gov/jpl/wise/pia17990/#.YxOWDlMo80N).


---

### Answer by username_6 on 2022-09-02. Score: 4

## Rogue Planets are Discovered Mainly Through Micro-Lensing

We have alreqdy discovered a number of rogue exoplanets, but we don't do it by directly imaging them. Rather, we detect them by watching their gravity distort the light of a star they pass across.

Data from the Kepler mission has been used to identify 27 rogue planet candidates, including four that may be earth-sized:

[Candidate short-duration events from the first space-based survey for planetary microlensing](https://academic.oup.com/mnras/article/505/4/5584/6315707?login=false)

The upcoming Nancy Roman Grace Telescope will survey the sky in the direction of the galactic bulge looking for micro-lensing events. It will use a wide-field camera to look at the same region of the galaxy repeatedly for transient events. It will be able to discover rogue planets down to the size of Mars.

I don't think the distance to us really matters much - it's more about the geometry between the rogue planet and the star it is lensing. This technique will also be used to detect Earth-sized exoplanets around other stars.

[Nancy Roman Grace Galactic Bulge Time Domain Survey](https://roman.gsfc.nasa.gov/galactic_bulge_time_domain_survey.html)

> **Comments:**
> **username_7**: Good info. - I wasn't aware of this work. I suppose the deflections are far too small to measure.  I would be nice to have a deflection curve as well as a light curve.  
> **username_4**: How does one make the difference between a rogue planet and a regular exoplanet transit? Especially in the case of an exoplanet with a very long orbit where we wouldn’t detect regularly repeating transits?  

---

