How do we have telescopes that are powerful enough to see nearby galaxies, such as Andromeda (M31), but are unable to see TRAPPIST-1 or the other exoplanets in its solar system in detail when TRAPPIST-1 is only 39 light years away?

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    $\begingroup$ You realise that 1) A galaxy is a lot bigger than dwarf star? 2) A galaxy is a lot brighter than an ultra-cool, dwarf star? 3) A flea close by in the evening is a lot harder to see than an elephant a mile away at noon? 4) The only reason to start a thread on this Q&A site is because you have a question? $\endgroup$ Feb 25, 2017 at 16:08
  • $\begingroup$ Yes. And because I want answers, asking questions is what got us this far. Why stop now? $\endgroup$ Feb 25, 2017 at 16:12
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    $\begingroup$ "Why stop now?" Well certainly stop adding "I have a question" as the title of the question. If everyone thought like that, there'd be a single title for every thread across every stack exchange site! Titles should be descriptive, so that people browsing the question list can easily tell if they have the knowledge needed to answer them, or enough interest to read them. $\endgroup$ Feb 25, 2017 at 16:16
  • $\begingroup$ Why does it matter how I ask my question or how research information? $\endgroup$ Feb 25, 2017 at 16:20
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    $\begingroup$ "Why does it matter how I ask my question" If it's not clear by now that giving a question an uninformative title is counter-productive, then I just don't know how to explain it! "..or how research information?" Ahh.. but that part I can expand on. On the upper left is a number with a upward pointing triangle above it, and a downward pointing triangle below. People also use that to tell if a question is worth reading or replying to. The downward pointing triangle has a tool-tip that reads "This question does not show any research effort, it is unclear or not useful.". $\endgroup$ Feb 25, 2017 at 16:25

3 Answers 3


How do we have telescopes that are powerful enough to see nearby galaxies, such as Andromeda (M31), but are unable to see TRAPPIST-1 or the other exoplanets in its solar system in detail when TRAPPIST-1 is only 39 light years away?

You don't need a telescope to see M31. It has an apparent magnitude of 3.4, which means it is easily visible to the naked eye, perhaps even in cities. The reason is that the Andromeda Galaxy comprises about a trillion stars that collectively emit about 26 billion times as much visible radiation as does the Sun.

Compare that to TRAPPIST-1 (the star, not the planets). This is an extremely dim star, about as small and dim as stars can get. TRAPPIST-1 has an apparent magnitude of 18.8, which means that it is inaccessible to all but the most powerful of telescopes. It emits about 1/260000th of the visible radiation emitted by the Sun. Another way to look at it: Andromeda emits about 7 quadrillion ($7\times10^{15}$) more light than does TRAPPIST-1.

With regard to seeing the exoplanets that orbit that star, the furthest out exoplanet orbits TRAPPIST-1 at a radius of about 0.063 astronomical units. At 39.5 light years away, that means a maximum separation of 0.005 arc seconds between the star and the exoplanet. The Hubble Space Telescope's resolution is about 0.1 arc seconds. TRAPPIST-1 and all seven of its exoplanets would resolve as a single pixel.


Trappist-1 is difficult to see because it's not very big. In fact it's so small (~11% the radius, and ~8% the mass of the Sun), that if it were any less massive, it wouldn't be able to support fusion!

Also, the human eye isn't very good at seeing much of the EM spectrum. So it's not emitting much visible light, BUT we can build tools to detect the infrared light that Trappist-1 gives off.

Galaxies on the other hand are big. REALLY big. So big in fact that they contain somewhere on the order of 100 thousand million stars! And of these stars almost all of them are brighter than Trappist-1.

So we aren't really seeing those galaxies in more absolute detail, but since they are so much more bigger, we can see "intricacies" at the star-level


In recent years, astronomers have been able to see (in their words, 'image directly') a few exoplanets. This is one of those:


We have found about 2000 exoplanets so far, but we've only been able to directly image 22 of them as of 2017. Most exoplanets are found using indirect methods (e.g. by measuring the star's brightness over time, if there is a regular dip in brightness that may mean a planet has passed in front of the star).

It's difficult to directly image an exoplanet.

  • planets are very small compared to stars,
  • they are far less bright than stars,
  • and they are (when viewed from far away) very close to their star.

In the image above you can see a 'halo' around the star. This is an optical effect: the star's light gets smeared out to a larger diameter than it should be. For many exoplanets, this means the planet gets lost in this halo.

The smallest object you can see in a telescope is determined by the angular resolution of a telescope. Most planets are so close to their stars that they're below the angular resolution of even the largest telescopes.

We have spectacular images of nearby galaxies because they're really big. This is the Andromeda galaxy compared to the Moon:

Andromeda is huge

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    $\begingroup$ "This is the Andromeda galaxy compared to the Moon:" Crap that is one (visually) huge galaxy! $\endgroup$ Feb 26, 2017 at 9:29
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    $\begingroup$ What a shame it is not visible like this with human eye. $\endgroup$
    – jkavalik
    Feb 26, 2017 at 10:04

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