Using
command substitution, you can easily loop through the contents of a file. If the file will be more than one line, the only thing different you'll need to do is pipe the output of the file through
tr and replace all newlines with spaces.
This will only work, though, if your file does not contain words that have spaces in them.
Code:
filename=./planetlist.txt
for planet in `cat "$filename" | tr '\n' ' '`; do
echo $planet
done