Literally, in order for a vortex to be classified as a tornado, it must be in contact with the ground and the cloud base. Weather scientists haven't found it so simple in practice, however, to classify and define tornadoes (per this essay by Doswell).
Bikini Island, 2015. Credit: Emlyn Hughes. (Phys.org)âA team of researchers from Columbia University in New York has found that all of the Marshall Islands involved in nuclear tests by the U.S. are now habitable, except for Bikini Atoll.
The habitable zone is the region around a star where a planet orbits that is not too hot or too cold, allowing liquid water and potentially life to exist. The Kepler satellite works best when it comes to finding very large planets that orbit very close to their parent stars. The problem with these planets is that they tend to be way too hot to sustain life.