[katieHaylor (OP)]

William asks:

How do scientists know there aren't any planet-sized bodies in interstellar space? i.e. are they too far from the nearest star to be illuminated?

What do you think?

[Janus]

William asks:

How do scientists know there aren't any planet-sized bodies in interstellar space? i.e. are they too far from the nearest star to be illuminated?

What do you think?

They actually believe that there are many such planets in the galaxy. Some are expected to have formed without a star and other were ejected from their host star system.   We've already detected 18 possible candidates.  The estimates of just how many there are in the galaxy vary.
 While they themselves are too dim to see, they are located by micro-lensing.  Their gravity bends the path of light as it passes, which slightly distorts the background image of stars behind them.

[Catastrophe]

In answer to your question: How do scientists know there aren't any planet-sized bodies in interstellar space? i.e. are they too far from the nearest star to be illuminated?
here is an example of one believed to be roughly the size of Mars approximately 4.3 billion years ago which hit our poor little planet and created our Moon.
Luckily for us the formerly interstellar visitor has not yet brought any relatives to repeat the experience.

To be serious, whilst the basis of this reply is believed to be correct, there were many more large planetary-sized objects around at that time so the background scenario is not very similar to that of today, and you are correct in your proposition that any planet, by definition, would need to be illuminated. BUT WAIT just because we normally respond to electromagnetic waves of a limited range (what we call light) even we are starting to use apparati which respond to other wavelengths such as infrared or Xrays.

Let us hope that the crashing of sound vibrations as a long lost relative arrives searching for his or her distant relative, having arrived unseen from the direction of our star too quickly to be detected by our electromagnetic apparati
(Sorry if you don't like 'apparati' but I do prefer it to apparatuses)

Catastrophe

[Janus]

In answer to your question: How do scientists know there aren't any planet-sized bodies in interstellar space? i.e. are they too far from the nearest star to be illuminated?
here is an example of one believed to be roughly the size of Mars approximately 4.3 billion years ago which hit our poor little planet and created our Moon.
Luckily for us the formerly interstellar visitor has not yet brought any relatives to repeat the experience.

There is nothing to suggest that Theia (the proposed object which collided with the Earth) was interstellar in nature.  In fact, it is generally accepted that the collision was at a moderate velocity, leading to the conclusion that it formed inside the Solar system. ( an object falling inward from interstellar space would have to be moving at at least 42 km/sec by the time it got in as to close to the Sun as the Earth is)
One hypothesis is that it originally formed at either the L4 or L5 point, which are gravitational anomalies that are 60 degrees ahead of and behind the Earth in its orbit.  It was then perturbed by other some of the other junk left over from the formation of the Solar system (which was much more crowded then), until it drifted into a collision with the Earth.