Water vapor is not unusual on planets, and there is a large amount of water vapor in the atmospheres of the solar system, including gasplanets such as Jupiter, Saturn, Uranus and Neptune, but rare if they are Earth-like rocky planets. Despite the Earth itself, Venus has a thick atmosphere, but the proportion of water vapor is very small, and Mars, although there is a large amount of water ice in the polar cap, but because of the thin atmosphere, so the atmosphere is equally rare. This makes it the biggest obstacle to judging how many planets in the universe are suitable for the existence of carbon-based organisms similar to Earth. The latest major breakthrough was a team at the University of Montreal that claimed to determine that an Earth-like planet called K2-18b contains enough water vapor in its atmosphere by analyzing spectral changes in the pre-stage of another star. Add to this the fact that K2-18b is in the galaxy’s “habitable zone”, that is, in the temperature range where liquid water is possible, and perhaps K2-18b has its own water cycle. The problem, of course, is that we don’t have enough resolution to make more accurate and direct observations of it. Perhaps after the launch of the James Webb Space Telescope in 2021, we will have more accurate data to analyze the atmospheric composition of K2-18b and similar stars. For the time being, K2-18b can only be used as a dream, perhaps earth has oceans, clouds, rain planets, in the universe is not so rare and uncertain.