A recent science news item has caught the imagination of public: astronomers hypothesize the existence of a hitherto unseen planet that might be lurking in the vast space beyond Neptune. There are thousands of small icy bodies that stretch out beyond the orbit of Neptune from a distance of 30 AU all the way to 500 AU. They are known as the Kuiper Belt Objects (KPO). In recent years, astronomers have found that a few of these KPOs are clustered in a particular part of the solar system in a way that has a low probability of occurrence.
Six Kuiper Belt Objects cluster together in a way that cannot be explained by random juxtaposition. These particular KPOs, six to be exact, are quite far. Their perihelia (closest approach to the sun) are distributed between 150 and 500 astronomical unit (AU). We should remind ourselves that Earth is situated at a distance of one AU from the sun, Neptune's perihelion is about 30 AU and the current position of Pluto is 32.5 AU. One of the peculiarities of these KPOs is that they all achieve their perihelia when they cross the ecliptic. Astronomers' calculations suggest that a massive planet in a highly eccentric orbit and with a semi-major axis of 700 AU could herd these KPOs into orbits that follow the above criteria.
But the story of the ninth planet is only part of the planetary science revolution that is taking place now. The Kepler spacecraft has discovered thousands of planets around other stars in our immediate galactic neighborhood. However, most these planets have masses that exceed Earth's and sometimes even that of Jupiter; but more than that, these planets orbit their suns at much closer distances than the Earth's distance to the sun.
In the above figure, out of thousands of potential planets, only one matches Earth's parameters. It might not mean our solar system is unique, but it might indicate that it is rare. But how did our solar system escape a fate that is replete with giant Earths and giant Jupiters that are orbiting the sun at close ranges? In recent years, astronomers have devised a way--which would seem really fantastic-- to explain this. Their mathematical models state that, during the nascent state of our solar system, Jupiter was born at a distance of 6 AU from the sun. Over millions of years Jupiter migrated to the inner solar system, up to a distance of 1.5 AU, i.e., to the present Martian orbit.
In the wake of Jupiter's travel, all the planetesimals (small planetary bodies) in the inner solar system were stirred up, some of them were ejected out of the inner solar system, and most were sent into orbits that eventually crashed into each other. The leftover fragments dragged large planets that existed within the inner solar system into the sun. Thus, unlike the extrasolar systems discovered by the Kepler, our inner solar system was cleared of debris. In the meantime, Saturn came into existence and Jupiter went into an orbital resonance with Saturn. This resonance helped Jupiter to migrate outwards to its current position of 5.2 AU. During this outbound migration, Earth was formed from the leftover planetesimals. The entire process might have taken a few hundred million years.
Hence, Earth owes its existence to Jupiter, or to Saturn however you look at it. If it were not for Saturn, Jupiter would have settled in an orbit close to the sun and Earth would never have been born. Then our solar system would have looked like most of the systems that Kepler has found. The ninth planet that the astronomers are predicting could have been ejected from the inner solar system during Jupiter's migration.
All this undermines the nice solar system formation scenarios that are in our textbooks, but as we know that science progresses by rebuilding on restructured or discarded ideas.
A figure from Batygin, K. & Laughlin, G. 2015. It shows, during Jupiter's migration within the inner solar system, all planetesimals are either transported into orbits that caused their fragmentation or they were scattered outwards. The planetesimals, in turn, were responsible for the spiraling of the giant earth-like planets into the sun.