One often hears the question of which of the known planets is the largest. The planet of the solar system with the largest mass is Jupiter. However, in density, it is inferior to many planets. For example, the density of the Earth is four times larger. This fact allowed scientists to conclude that Jupiter consists mainly of gases, does not have a solid core. Also, Jupiter is the largest planet in the solar system in terms of the radius, and, accordingly, the volume, surface and other characteristics associated with the size.
If we include in this competition the size of the planet found in other star systems, the so-called "exoplanets," then Jupiter will turn out to be far from a record holder. For example, the planet TrES-4 is 1.4 times larger than the largest planet in the solar system. According to calculations, the gas cloud should be at least 15 times larger, so that the internal reactions of nuclear fusion begin. It is the presence of this process that distinguishes stars and planets.
New methods of observation allow astrophysicists to discover newer and newer planets near other stars. The results achieved in recent decades have shown that the solar system is only one of many planetary systems. With these researches the old hope of mankind to find other inhabited worlds is connected. The first exoplanet was discovered in 1992, and now several hundreds of exoplanets are known. Most of the known exoplanets are giants the size of Jupiter or more.
Planets that revolve around distant stars are extremely difficult to detect, since they do not radiate their own Light and are in close proximity to the central star of the corresponding system. To circumvent these difficulties, scientists use various methods that allow you to catch subtle effects that show the presence of a planet near a particular star. The most common method for finding planets from distant stars is to observe the modulations of radial velocity. This method is based on the fact that the planet has a small influence on the motion of the star, which can be caught with very accurate spectral measurements. This method is most likely to find the most massive planets that are too close to the star. The chances that these worlds will be inhabited are minimal. The greatest chance to find extraterrestrial life on terrestrial planets rotating in the belt, adapted to creating and sustaining life.
Unfortunately, the discovery of such planets presents an unusual difficulty for terrestrial telescopes. For this purpose, it is planned to launch orbital telescopes, the sensitivity of which will be sufficient to observe exoplanets of the terrestrial type.
One of such orbital observatories "Kepler" is able to detect exoplanets, the size comparable to the Earth and even less. For example, the planet Kepler-37b, found in the system in the constellation of Lyra, is comparable in size to the Moon. It is completely devoid of the atmosphere and is heated to great temperatures and the likelihood that it has life on it is not the greatest. The planet of the solar system, similar in characteristics with this exoplanet - Mercury. But the fact that Kepler-37b definitely consists of hard rocks is a wonderful and reassuring fact.