As mentioned in the Srimad Bhagavatam, the ancient rishis were able to perform this remarkable feat by meditation alone. It may have been done by meditating during the periods when the planets are transiting the natal Moon of the meditator. For the exoplanets, one would have to research the star that the exoplanet is revolving around and determine its placement in the rashi or constellation in the zodiac.
On a side note, a Shaiva guru from Kauai, Hawaii wrote a book a few years ago stating that ancestors of humans came from another star system and spiritually traveled the universe using the Sun as the guide to find Earth. They eventually found Earth and materialized on land through the fruits and vegetation that was offered by the natives of Lemuria.
