One of the easiest questions a person can ask is, "Are we alone in the Universe?" Everything we have learnt so far suggests that life could exist on millions or billions of other planets in our galaxy alone. There are a huge number of exoplanets (that's the name given to planets outside the solar system) orbiting other stars.
Throughout the universe, water, oxygen, carbon and other elements are abundant, without which life would not be possible, but modern technological devices have not yet been able to detect life on other planets. So let's take a closer look at exoplanets to better understand what they are and whether life is possible on them.
Types of exoplanets
Until 1995, the existence of exoplanets had not been proven, but today research has shown that almost every star has several planets orbiting it; in our Milky Way alone there are hundreds of billions, perhaps even trillions.
The science of exoplanets is still in its infancy, but in the coming years and decades the James Webb Space Telescope and the European Very Large Telescope are expected to expand our knowledge of exoplanets.
Like the planets in our solar system, exoplanets come in different sizes and structures, and the main types are listed below:
- rocky planets, or Earth-type planets, are common in other planetary systems, they are composed mainly of heavy elements such as silicon, oxygen or metals, and have no significant atmosphere;
- super-Earths are planets 1-10 times the mass of the Earth, they may well be habitable, they may contain water, and they often experience earthquakes;
- water planets - their surface is almost entirely covered by the ocean, and it seems plausible that life could develop on oceanic planets as well;
- desert planets - planets without water on their surface, like Mars;
- gas giants are planets with a mass 10 times or more above Earth's, consisting mostly of hydrogen and helium, like Jupiter, Saturn, Uranus and Neptune;
- hot Jupiters, or pegasids, are gas giants orbiting their home star in a close orbit, with a radius of less than 0.5 astronomical units;
- solitary, or wandering, planets are planets without a central star, they drift around our Galaxy, they do not glow themselves and are very hard to detect.
Exoplanets are too far from Earth to be seen with even modern telescopes from Earth, but methods do exist to detect them.
The radial velocity method (Doppler method) is based on the fact that both the star and the planet revolve around a common centre of gravity. This means that the star never stays completely still, it rotates around the common centre of gravity of the entire star system, which can be detected from Earth using the Doppler effect.
The transit method is based on the star losing some of its luminosity when a planet passes in front of it. The Kepler Space Telescope has continuously measured the luminosity of 145 000 stars over several years and has detected thousands of exoplanets.
How many exoplanets have been discovered to date?
More than 4,000 exoplanets have been discovered to date. No one knows how many exoplanets there are in the entire universe. Based on probability theory, the total number of planets in our Milky Way alone is more than 10 trillion, while in the observable Universe there are about 10 to the power of 24 - that's a quadrillion!