The mass of a proton is 1.673 x 10^(-27) (That is 10
cube with -27)
I tried Special Characters without any luck.
The previous replies contain the easiest solutions. Use them. However, you should also be able to do it using the Character Palette.
The trick is to expand the Character Info section - then highlight a number in Pages and either drag it to the Character Info field or use the wheel menu in the lower left corner of the Character Palette and choose "Show Character selected in Application".
Then you will see all variants there are of the different numbers, including superscripts, subscripts and Chinese versions.
For example for the number 5, I get the following variants: 5⁵₅⑤⑸⒌⓹❺㈤㊄㐅五伍5. The second one is the superscript.
The number you want to write can then be written as 1.673 x 10⁻²⁷.
The disadvantage with the Character Palette solution is that not necessarily all superscript letters are defined in the font you selected, so you may end up with a mix, which looks strange. Depending on your browser and browser settings, 10⁻²⁷ may not look very well aligned on this page. However, if you paste it into Pages and select for example Hoefler as font, it should look fine.
So why use it? The advantage is that you end up with plain text, which is easy to copy between applications and operating systems regardless of file format. In case you need to do that.