We just found stronger evidence for yet another planet. It was a super-Earth that fell into the Sun when the solar system was young.
This is in addition to the Neptune-sized planet that was between Saturn and Uranus and was ejected.Planets initially form in the protoplanetary disk from gas and dust, but as they grow larger, more and more massive rocks and eventually planetoids collide to contribute to their mass. This process ended for Earth when the Mars-sized planet Theia collided with it, forming the Moon.
There might also have been a huge world the size of Neptune between the orbits of Saturn and Uranus, and it was ejected. Some earlier studies linked this event to the Late Heavy Bombardment around 3.8 billion years ago. In this model, ejecting this planet disrupted asteroids and comets, which rained down on the remaining worlds, including Earth. However, more recent research suggests the ejection may have happened earlier and may not have been linked to the Late Heavy Bombardment.
This month, computer simulations suggest that the Sun swallowing a super-Earth planet within less than 50 million years after the solar system formed could explain some known anomalies of our star.
The lost planet might have been between 5 and 10 times as massive as Earth, and it enriched the Sun with heavy elements just below the present-day convection zone. This explains its helioseismicity, unusually low lithium abundance, and convection-zone depth. Computer simulations that tried to explain these issues by enriching the Sun with just dust couldn’t reproduce what is observed. Only swallowing a super-Earth planet agrees with these characteristics of our Sun.
Furthermore, we already know that close to 5000 systems with exoplanets and super-Earths are frequent in close orbits to their stars. The existence of a swallowed planet then confirms that our system isn’t unusual; it simply lost such a world early in its history.