No, not all planets have an ionosphere.
Based on the provided reference, all the planets in the solar system, except for Mercury, have ionospheres. These ionospheres, except for Pluto's, have been observed either in-situ (directly) or remotely.
Here's a summary:
| Planet | Has Ionosphere? | Observation Method |
|---|---|---|
| Mercury | No | Not Observed |
| Venus | Yes | In-situ/Remotely |
| Earth | Yes | In-situ/Remotely |
| Mars | Yes | In-situ/Remotely |
| Jupiter | Yes | In-situ/Remotely |
| Saturn | Yes | In-situ/Remotely |
| Uranus | Yes | In-situ/Remotely |
| Neptune | Yes | In-situ/Remotely |
| Pluto | Yes | Not Observed In-situ/Remotely |
Therefore, while most planets within our solar system are confirmed to have an ionosphere, there are exceptions like Mercury. Further studies might uncover more details about planetary ionospheres in the future.
