<p>We can't <a href="https://astronomy.stackexchange.com/questions/51903/is-there-enough-data-in-light-coming-from-distant-exoplanets-for-earth-orbiting">directly image an exoplanet</a>. Most of the time we detect planets by occlusion of the stars light. However, we <em>occasionally</em> can filter out the star's light and measure the light from the planet directly. Although we only see a single point of light, can we form an image by observing brightness over time?</p>
<p><strong>No axil tilt</strong> As the planet rotates (assuming it rotates) we can measure a days brightness variation and get the albedo of each longitude &quot;strip&quot;. This gives a 1D image.</p>
<p><strong>Axial tilt</strong>. We can use seasonal variations of albedo to get latitude information and combine it with daily rotation to get a 2D image.</p>
<p><strong>Specular reflections</strong>. If the planet has oceans and the angles are just right we may glimpse a <a href="https://www.planetary.org/space-images/specular-reflection-off-of-a-titan-lake" rel="nofollow noreferrer">refection</a>. It would fade in or out dependent on clouds and terrain (if it has such features). As a bonus this light would be polarized. But would the specular reflection be a detectable fraction of the planet's light?</p>
<p><strong>Eclipses</strong>. If the planet has a large moon the eclipses would affect the amount of light reflected.</p>
<p>Are any of these methods feasible in the near future? What kind of image resolution should we expect for an Earth-like planet around a sun-like star 100 light years away?</p>

CC BY-SA 4.0
2023-02-15T09:03:24.823Z
There is an entire class of exoplanets discovered by direct imaging, check e.g. the HR 8799 system. It's just spatially unresolved direct imaging. Furthermore, using occultations (the opposite of transits) in thermal imaging, you can create 'heat maps' of Hot Jupiters, such as HD 189733b. Take those techniques in mind, when you ask what you ask.
AtmosphericPrisonEscape
CC BY-SA 4.0
2023-02-16T23:47:19.913Z
@AtmosphericPrisonEscape: I had HR 8799 in mind. The general idea is if you carefully measured the brightness of one of those planets as it rotated you may get some sort of image (either by thermal or reflected light) so long as the poles weren't too much aligned with us. Rocky planets have a fixed terrain so longer-term measurements could yield a sharper image.

Occultations could also get an image as you state. Probably only with thermal radiation and not visible light for an Earth-like planet (visible would be too drowned out)?
Kevin Kostlan
CC BY-SA 4.0
2023-02-17T00:36:51.433Z
The planets in HR 8799 are gas giants, so I'd be very surprised so see any axial tilt and any variation with time in the IR and VIS. Rocky planets with occultations are improbable to happen even with the ELT's, as the occultation signals are an order of magnitude smaller than the transit signals, and small exoplanet transits are going to be barely doable with the next Gen instruments. Furthermore, tidal locking will be a problem for the close-in planets, as you will see them at always the same phase.
AtmosphericPrisonEscape
<exoplanet>
Imaging an exoplanet via brightness over time
Kevin Kostlan
# Imaging an exoplanet via brightness over time

(Asked by: username_0 on 2023-02-15)

We can't [directly image an exoplanet](https://astronomy.stackexchange.com/questions/51903/is-there-enough-data-in-light-coming-from-distant-exoplanets-for-earth-orbiting). Most of the time we detect planets by occlusion of the stars light. However, we *occasionally* can filter out the star's light and measure the light from the planet directly. Although we only see a single point of light, can we form an image by observing brightness over time?

**No axil tilt** As the planet rotates (assuming it rotates) we can measure a days brightness variation and get the albedo of each longitude "strip". This gives a 1D image.

**Axial tilt**. We can use seasonal variations of albedo to get latitude information and combine it with daily rotation to get a 2D image.

**Specular reflections**. If the planet has oceans and the angles are just right we may glimpse a [refection](https://www.planetary.org/space-images/specular-reflection-off-of-a-titan-lake). It would fade in or out dependent on clouds and terrain (if it has such features). As a bonus this light would be polarized. But would the specular reflection be a detectable fraction of the planet's light?

**Eclipses**. If the planet has a large moon the eclipses would affect the amount of light reflected.

Are any of these methods feasible in the near future? What kind of image resolution should we expect for an Earth-like planet around a sun-like star 100 light years away?

**Tags:** exoplanet
> **Comments:**
> **username_1**: There is an entire class of exoplanets discovered by direct imaging, check e.g. the HR 8799 system. It's just spatially unresolved direct imaging. Furthermore, using occultations (the opposite of transits) in thermal imaging, you can create 'heat maps' of Hot Jupiters, such as HD 189733b. Take those techniques in mind, when you ask what you ask.  
> **username_0**: @AtmosphericPrisonEscape: I had HR 8799 in mind. The general idea is if you carefully measured the brightness of one of those planets as it rotated you may get some sort of image (either by thermal or reflected light) so long as the poles weren't too much aligned with us. Rocky planets have a fixed terrain so longer-term measurements could yield a sharper image.  Occultations could also get an image as you state. Probably only with thermal radiation and not visible light for an Earth-like planet (visible would be too drowned out)?  
> **username_1**: The planets in HR 8799 are gas giants, so I'd be very surprised so see any axial tilt and any variation with time in the IR and VIS. Rocky planets with occultations are improbable to happen even with the ELT's, as the occultation signals are an order of magnitude smaller than the transit signals, and small exoplanet transits are going to be barely doable with the next Gen instruments. Furthermore, tidal locking will be a problem for the close-in planets, as you will see them at always the same phase.  

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