Most of them are known since ancient and the last discovered, called Pluto, was discovered in the 1930s. Many astronomers believe that our Solar System has at least another planet, even if it has not been discovered. Several attempts will be made to find planet X using the Pioneer 10 space probe.
Pioneer 10
Jupiter was launched in March 1972 with the main objective of studying the asteroid belt. It is currently 7.7 billion kilometers from the Sun (45 astronomical units) and five years ago exceeded the limit of the known Solar System.
Radio waves moving through the speed of light take 12 hours and 26 minutes to get from the Earth to the probe. Researchers at NASA's Ames Research Center in California take control of the ship and believe the signal emission will last ten more years.
It will be the first human object that will abandon our system and if in the future find other intelligent beings, carries in the fuselage a plate that indicates who we are.
Pioneer 10
The solar wind is still being measured (charged particles dragging the sun). Scientists do not know the extent of the solar wind. They know that it extends to a distance of 45 astronomical units and that the limit can be between 50 and 100 astronomical units. Therefore, they expect Pioneer 10 to be able to detect this limit.
Last December, the probe has been conducting a series of gravity wave detection sessions (we do not know the results yet). Gravity waves were predicted by Einstein when it came to raising the theory of relativity, but they have never been found.
Pioneer 10 will also participate in the detection of planet X. As speculated, this planet is about 100 astronomical units of the Sun and occupies a 1,000-year orbit around our star. This planet is responsible for the disturbances observed in the orbits of Uranus and Neptune. The mass of the planet would be 1-4 times greater than that of the Earth and its diameter would be around 14,000 km.
After the visit to Pioneer 11, Jupiter and Saturn, launched in 1973, there are currently 25 astronomical units, and although intended to be used to detect gravity waves and the tenth planet, communication has not been considered to be maintained for as long as with the Pioneer 10.