While visiting the planet in 2151, Captain Jonathan Archer and the crew of the Enterprise NX-01 came upon the Eska hunters. One of the Wraiths appeared to Archer as a woman he recognized; she had read his mind and taken the form of the "glimmering girl" in Yeats's The Song of the Wandering Aengus as he had imagined her when he was a child. She explained that her people were shapeshifters, and the Eskans hunted them using sensors to track them by identifying the Wraiths' chemical signature they give off when they are afraid. Dr. Phlox made a compound that masked their chemical signature concealing them from the hunters and hid them from the Eska's sensors.
Once her people had protected themselves with Phlox's compound, Archer's mysterious friend thanked him and let him see her true form, commenting to him as she departed that she had taken that form to remind him to never stop searching for what seemed to be unattainable.
The shapeshifting powers of a Wraith are very formidable. For example, if they become a rock then sensors will be unable to tell the difference. This would explain why Subcommander T'Pol was unable to detect any lifesigns on the planet. (ENT: "Rogue Planet")