<p>I'm pretty sure it's restricted to the near-IR, with the shortest wavelengths being <span class="math-container">$Y$</span>-band or <span class="math-container">$J$</span>-band (i.e., 1 or 1.2 microns) and the longest being <span class="math-container">$L$</span>-band (i.e., 3.8 microns). This is probably because 1) classical adaptive optics systems use the optical for corrections that are applied in the near-IR (for reasons I discussed in <a href="https://astronomy.stackexchange.com/questions/14795/why-arent-ground-based-observatories-using-adaptive-optics-for-visible-waveleng/14825#14825">this answer</a>); and 2) young, hot planets may radiate enough in the near-IR to be brighter relative to the star than they would be in the optical.</p>
<p>&quot;Fomalhaut b&quot; used to be an example of an exoplanet imaged (with the <em>Hubble Space Telescope</em>) in the optical, with wavelengths as short as <span class="math-container">$\sim 0.4$</span> microns, but the consensus now seems to be that's it's not a planet at all, and is instead something like an expanding cloud of dust from the collision of two protoplanetary objects.</p>

CC BY-SA 4.0
2021-10-13T10:33:58.647Z
2021-10-13T10:33:58.647Z
Peter Erwin
<p>Phys.org's <a href="https://phys.org/news/2021-10-radio-distant-stars-hidden-planets.html" rel="nofollow noreferrer">Radio signals from distant stars suggest hidden planets</a> summarizes the Nature Astronomy paper <a href="https://www.nature.com/articles/s41550-021-01483-0" rel="nofollow noreferrer">The population of M dwarfs observed at low radio frequencies</a> (also in <a href="https://arxiv.org/abs/2110.03713" rel="nofollow noreferrer">arXiv</a>) but this is an indirect measurement.</p>
<p>Here I'd like to ask about techniques that provide <em>resolved imaging</em> of the system, where the exoplanet can be separated spatially from its primary. In other words some kind of image where one can say &quot;here's the exoplanet, north-east of the primary at X micro-arc seconds.&quot;</p>
<p><strong>Question:</strong> What are the shortest and longest wavelengths at which exoplanets have been resolved from their primaries?</p>
<p>I think these are usually done using adaptive optics in red or near infrared light and silicon CCDs, so perhaps something like 700 to 1000 nm. But has any exoplanet been imaged in visible light? Or UV, perhaps from the HST?</p>
<p>On the other side, have large radio telescope arrays ever resolved exoplanets? I'd guess not as their Suns would not illuminate them with sufficient brightness (<a href="https://astronomy.stackexchange.com/q/32606/7982">How far have individual stars been seen by radio telescopes?</a>)</p>
<hr />
<p>This is a particularly nice example <a href="https://en.wikipedia.org/wiki/File:HR_8799_Orbiting_Exoplanets.gif" rel="nofollow noreferrer">source</a>:</p>
<blockquote>
<p>Motion interpolation of seven images of the HR 8799 system taken from the W. M. Keck Observatory over seven years, featuring four exoplanets</p>
</blockquote>
<p><a href="https://i.sstatic.net/fXYno.gif" rel="nofollow noreferrer"><img src="https://i.sstatic.net/fXYno.gif" alt="Motion interpolation of seven images of the HR 8799 system taken from the W. M. Keck Observatory over seven years, featuring four exoplanets" /></a></p>

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2021-10-12T14:21:23.860Z
I'd think this one could be resolved with an exhaustive look at all the planet's we've found by direct image, which wouldn't be all that exhaustive since we've only imaged a small fraction of the ones we've found.
zephyr
CC BY-SA 4.0
2021-10-12T14:46:52.783Z
@zephyr I'd thought it would be the other way around; that nobody would dedicate the observing time necessary to interferometrically image random stars in hopes there would be a resolvable planet. Wouldn't they comb the list of known systems discovered by parallax or spectroscopically for the best candidates for imaging, then use that observing time on a target that's very likely to be successfully resolved?
uhoh
CC BY-SA 4.0
2021-10-12T20:53:36.007Z
Yeah, that's how they'd do it. I just mean that, the number of exoplanets that have been directly imaged is very small. Since you seem to be interested in the range of wavelengths used in imaging exoplanets, and since the number of imaged exoplanets is so small, it shouldn't be too much work to just find all imaged exoplanets and look at the range of the very small set.
zephyr
CC BY-SA 4.0
2021-10-12T21:16:41.967Z
@zephyr ah, got it.
uhoh
CC BY-SA 4.0
2021-10-13T10:42:04.503Z
@uhoh In fact, the Gemini Planet Imager Exoplanet Survey targeted $\sim$600 young stars (without pre-existing planet detections by other means) specifically in hopes of finding planets. (https://arxiv.org/abs/1904.05358)
Peter Erwin
CC BY-SA 4.0
2021-10-13T10:44:27.173Z
Planets detected by spectroscopy (radial velocity measurements) or occultations would *not* be good candidates for direct imaging, because they would be too close to their host stars.
Peter Erwin
CC BY-SA 4.0
2021-10-13T22:43:46.583Z
@PeterErwin yes I'm starting to see what you mean...
uhoh
<observational-astronomy><exoplanet><radio-astronomy><hubble-telescope>
Shortest and longest wavelengths at which exoplanets have been resolved from their primaries?
uhoh
# Shortest and longest wavelengths at which exoplanets have been resolved from their primaries?

(Asked by: username_0 on 2021-10-12)

Phys.org's [Radio signals from distant stars suggest hidden planets](https://phys.org/news/2021-10-radio-distant-stars-hidden-planets.html) summarizes the Nature Astronomy paper [The population of M dwarfs observed at low radio frequencies](https://www.nature.com/articles/s41550-021-01483-0) (also in [arXiv](https://arxiv.org/abs/2110.03713)) but this is an indirect measurement.

Here I'd like to ask about techniques that provide *resolved imaging* of the system, where the exoplanet can be separated spatially from its primary. In other words some kind of image where one can say "here's the exoplanet, north-east of the primary at X micro-arc seconds."

**Question:** What are the shortest and longest wavelengths at which exoplanets have been resolved from their primaries?

I think these are usually done using adaptive optics in red or near infrared light and silicon CCDs, so perhaps something like 700 to 1000 nm. But has any exoplanet been imaged in visible light? Or UV, perhaps from the HST?

On the other side, have large radio telescope arrays ever resolved exoplanets? I'd guess not as their Suns would not illuminate them with sufficient brightness ([How far have individual stars been seen by radio telescopes?](https://astronomy.stackexchange.com/q/32606/7982))

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This is a particularly nice example [source](https://en.wikipedia.org/wiki/File:HR_8799_Orbiting_Exoplanets.gif):

> Motion interpolation of seven images of the HR 8799 system taken from the W. M. Keck Observatory over seven years, featuring four exoplanets

[![Motion interpolation of seven images of the HR 8799 system taken from the W. M. Keck Observatory over seven years, featuring four exoplanets](https://i.sstatic.net/fXYno.gif)](https://i.sstatic.net/fXYno.gif)

**Tags:** observational-astronomy, exoplanet, radio-astronomy, hubble-telescope

> **Comments:**
> **username_1**: I'd think this one could be resolved with an exhaustive look at all the planet's we've found by direct image, which wouldn't be all that exhaustive since we've only imaged a small fraction of the ones we've found.  
> **username_0**: @username_1 I'd thought it would be the other way around; that nobody would dedicate the observing time necessary to interferometrically image random stars in hopes there would be a resolvable planet. Wouldn't they comb the list of known systems discovered by parallax or spectroscopically for the best candidates for imaging, then use that observing time on a target that's very likely to be successfully resolved?  
> **username_1**: Yeah, that's how they'd do it. I just mean that, the number of exoplanets that have been directly imaged is very small. Since you seem to be interested in the range of wavelengths used in imaging exoplanets, and since the number of imaged exoplanets is so small, it shouldn't be too much work to just find all imaged exoplanets and look at the range of the very small set.  
> **username_0**: @username_1 ah, got it.  
> **username_2**: @username_0 In fact, the Gemini Planet Imager Exoplanet Survey targeted $\sim$600 young stars (without pre-existing planet detections by other means) specifically in hopes of finding planets. (https://arxiv.org/abs/1904.05358)  
> **username_2**: Planets detected by spectroscopy (radial velocity measurements) or occultations would *not* be good candidates for direct imaging, because they would be too close to their host stars.  
> **username_0**: @PeterErwin yes I'm starting to see what you mean...  

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## 1 answers

### Answer by username_2 on 2021-10-13. Score: 3 | Accepted answer

I'm pretty sure it's restricted to the near-IR, with the shortest wavelengths being $Y$-band or $J$-band (i.e., 1 or 1.2 microns) and the longest being $L$-band (i.e., 3.8 microns). This is probably because 1) classical adaptive optics systems use the optical for corrections that are applied in the near-IR (for reasons I discussed in [this answer](https://astronomy.stackexchange.com/questions/14795/why-arent-ground-based-observatories-using-adaptive-optics-for-visible-waveleng/14825#14825)); and 2) young, hot planets may radiate enough in the near-IR to be brighter relative to the star than they would be in the optical.

"Fomalhaut b" used to be an example of an exoplanet imaged (with the *Hubble Space Telescope*) in the optical, with wavelengths as short as $\sim 0.4$ microns, but the consensus now seems to be that's it's not a planet at all, and is instead something like an expanding cloud of dust from the collision of two protoplanetary objects.


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