<p>As we haven't measured the atmosphere of many rocky planets yet, there aren't many examples. But I found 55 Cancri e:
It is a super-earth with very close tidally locked orbit and an atmosphere that has been measured by Hubble. A year on the exoplanet lasts for only 18 hours and temperatures on the surface are thought to reach around 2000 degrees Celsius. The detected atmosphere seems to consists mainly of hydrogen and helium. So you do have a constant Twilight Zone, just a rather hot one. </p>

<p><a href="https://i.sstatic.net/zSQTP.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/zSQTP.jpg" alt="Poster:"></a></p>

<p>Sources: 
- <a href="https://asd.gsfc.nasa.gov/blueshift/index.php/2013/07/12/jasons-blog-next-stop-diamond-planets/" rel="nofollow noreferrer">Beeing tidally locked</a>
- <a href="https://phys.org/news/2016-02-super-earth-atmosphere.html" rel="nofollow noreferrer">Detection of the Atmosphere</a></p>

CC BY-SA 4.0
2019-07-18T13:17:40.450Z
Strange that there don't seem to be any guesstimates of a temperature difference between the day side and the night side.  Perhaps this rock has high enough thermal conductivity that it's reached uniform thermal equilibrium?
Carl Witthoft
CC BY-SA 4.0
2019-07-18T13:37:32.723Z
@Carl Witthoft "The "cold" side is still quite toasty by Earthly standards, with an average of 2,400 to 2,600 degrees Fahrenheit (1,300 to 1,400 Celsius), and the hot side averages 4,200 degrees Fahrenheit (2,300 Celsius). The difference between the hot and cold sides would need to be more extreme if there were no atmosphere." -[nasas.gov](https://www.nasa.gov/feature/jpl/lava-or-not-exoplanet-55-cancri-e-likely-to-have-atmosphere)
SpaceCore
CC BY-SA 4.0
2019-07-18T09:06:56.070Z
2019-07-18T09:06:56.070Z
SpaceCore
<p>If not, is there any reason such a planet couldn't exist? </p>

<p>I ask only because that planet would literally have a twilight zone and I want to know that that's a thing somewhere. </p>

CC BY-SA 4.0
2019-07-17T17:46:19.340Z
Planet Mercury **has** an atmosphere.
Carl Witthoft
CC BY-SA 4.0
2019-07-18T13:14:04.527Z
@mistertribs  true enough --  Asimov complained bitterly when it was disovered that Mercury is not tidally locked (screwing up a previous story).  But in the short term, one could observe a TwilightZone, tho' I agree not permanent.
Carl Witthoft
CC BY-SA 4.0
2021-12-21T17:59:42.280Z
@CarlWitthoft If you consider Mercury to have an "atmosphere" you should consequently recognize the lunar Apollo flights as the only human spaceflights in history. Jim Lovell, Frank Borman and Bill Anders were the first people in space then.
Giovanni-Reinstate Ceres Pluto
CC BY-SA 4.0
2021-12-21T21:38:36.923Z
@Giovanni-ReinstateCeresPluto I know you're having fun, but the various definitions of the start of "space" from Earth's surface are based more on relative atmospheric density than absolute.
Carl Witthoft
CC BY-SA 4.0
2021-12-22T07:01:26.320Z
@CarlWitthoft No I don't make fun of you. If you insist on calling surface-bound exospheres "atmospheres" then the Apollo moon flights are the only human flights that actually left the Earth's atmosphere, all other "spaceflights" would in fact be atmospheric flights. The Earth's atmosphere/exosphere extends to at least 6,200 mi above sea level, the highest orbital flight (Gemini 11) reached an apogee around 800 mi. Your consideration is still more reasonable than the 100-km-boundary. But according to the latter or the 50-mi-boundary, the surface of Mercury is well in outer space.
Giovanni-Reinstate Ceres Pluto
<rotation><planetary-atmosphere>
Do we know of any tidally-locked planets with atmospheres?
user28747
# Do we know of any tidally-locked planets with atmospheres?

(Asked by: username_0 on 2019-07-17)

If not, is there any reason such a planet couldn't exist?

I ask only because that planet would literally have a twilight zone and I want to know that that's a thing somewhere.

**Tags:** rotation, planetary-atmosphere
> **Comments:**
> **username_1**: Planet Mercury **has** an atmosphere.  
> **username_1**: @mistertribs  true enough --  Asimov complained bitterly when it was disovered that Mercury is not tidally locked (screwing up a previous story).  But in the short term, one could observe a TwilightZone, tho' I agree not permanent.  
> **username_2**: @CarlWitthoft If you consider Mercury to have an "atmosphere" you should consequently recognize the lunar Apollo flights as the only human spaceflights in history. Jim Lovell, Frank Borman and Bill Anders were the first people in space then.  
> **username_1**: @Giovanni-ReinstateCeresPluto I know you're having fun, but the various definitions of the start of "space" from Earth's surface are based more on relative atmospheric density than absolute.  
> **username_2**: @CarlWitthoft No I don't make fun of you. If you insist on calling surface-bound exospheres "atmospheres" then the Apollo moon flights are the only human flights that actually left the Earth's atmosphere, all other "spaceflights" would in fact be atmospheric flights. The Earth's atmosphere/exosphere extends to at least 6,200 mi above sea level, the highest orbital flight (Gemini 11) reached an apogee around 800 mi. Your consideration is still more reasonable than the 100-km-boundary. But according to the latter or the 50-mi-boundary, the surface of Mercury is well in outer space.  

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## 1 answers

### Answer by username_3 on 2019-07-18. Score: 3

As we haven't measured the atmosphere of many rocky planets yet, there aren't many examples. But I found 55 Cancri e:
It is a super-earth with very close tidally locked orbit and an atmosphere that has been measured by Hubble. A year on the exoplanet lasts for only 18 hours and temperatures on the surface are thought to reach around 2000 degrees Celsius. The detected atmosphere seems to consists mainly of hydrogen and helium. So you do have a constant Twilight Zone, just a rather hot one.

[![Poster:](https://i.sstatic.net/zSQTP.jpg)](https://i.sstatic.net/zSQTP.jpg)

Sources:
- [Beeing tidally locked](https://asd.gsfc.nasa.gov/blueshift/index.php/2013/07/12/jasons-blog-next-stop-diamond-planets/)
- [Detection of the Atmosphere](https://phys.org/news/2016-02-super-earth-atmosphere.html)

> **Comments:**
> **username_1**: Strange that there don't seem to be any guesstimates of a temperature difference between the day side and the night side.  Perhaps this rock has high enough thermal conductivity that it's reached uniform thermal equilibrium?  
> **username_3**: @Carl Witthoft "The "cold" side is still quite toasty by Earthly standards, with an average of 2,400 to 2,600 degrees Fahrenheit (1,300 to 1,400 Celsius), and the hot side averages 4,200 degrees Fahrenheit (2,300 Celsius). The difference between the hot and cold sides would need to be more extreme if there were no atmosphere." -[nasas.gov](https://www.nasa.gov/feature/jpl/lava-or-not-exoplanet-55-cancri-e-likely-to-have-atmosphere)  

---

