The formation of planets around a young star occurs within a disk of gas and dust encircling the young star called a protoplanetary disk. This results in a close alignment between the rotation axis of the star and the orbital motion of the planets after they have formed. However, measurements of the relative spin-orbit alignment of transiting extrasolar planets via the Rossiter-McLaughlin effect have shown that a number of these planets have orbits that are significantly misaligned with the rotation axes of their host stars. Gravitational interactions with other planets or the Kozai mechanism are two proposed means in which planets in initially aligned orbits can get perturb into misaligned orbits. A recently published paper entitled “Misaligned and Alien Planets from Explosive Death of Stars” proposes an alternative mechanism to explain the planets that are observed to be in misaligned orbits around their host stars.
In this paper, planets whose orbits are misaligned with the rotation axes of their host stars are suggested to have formed from high-speed blobs of gas that are produced in supernova remnants and planetary nebulae. Supernova remnants are formed following the death of massive stars in supernova explosions while planetary nebulae are formed from the death throes of Sun-like stars as they shed off their outer layers into space. These blobs of gas have been observed in great numbers around supernova remnants and planetary nebulae. High resolution images of young supernova remnants and planetary nebulae have shown that each of them is surrounded by thousands of blobs of gas that have cometary-like appearance possibly shaped by overtaking winds. As these blobs of gas travel through interstellar space, they sweep up ambient matter along the way, causing them to increase in mass and decelerate. Over time, these blobs of gas gradually cool by emitting radiation. Once the blobs of gas become sufficiently massive and cool, self-gravity takes over and cause the blobs of gas to collapse gravitationally to form gas giant planets.
Regardless of whether these blobs of gas eventually contract gravitationally to form gas giant planets, they can explain the considerable number of planets that are found to be in misaligned orbits around their host stars through a number of different ways. For a blob of gas that has already collapsed gravitationally to form a gas giant planet, it can be captured into a misaligned orbit around a star or perturb the original planets that have previously formed around a star into misaligned orbits. On the other hand, an uncollapsed blob of gas can be captured into a misaligned orbit around a star to form a misaligned disk of gas and dust from which the in situ formation of planets with misaligned orbits can occur. Furthermore, uncollapsed free-floating blobs of gas can also be captured by stars and strongly perturb their planetary systems.
Around young supernova remnants and planetary nebulae, a typical blob of gas has a similar mass as the Earth and a ‘head’ which spans tens of billions of kilometers across. Shaped into cometary-like morphologies by faster overtaking winds, a typical blob of gas travels with an average projected velocity on the order of a few hundred kilometers per second outwards from its parent supernova remnant or planetary nebula. Blobs of gas in young supernova remnants can be classified into two populations distinguished by their velocities where the low velocity population formed in the ejecta of the progenitor star before its supernova explosion while the high velocity population formed following the supernova explosion.
As the blobs of gas travel outwards, they cool by radiation and grow in mass by accreting gas and dust in the interstellar medium. The accretion process causes the blobs of gas to decelerate, making the blobs of gas or the eventual gas giant planet slow enough to be gravitationally captured during sufficiently close encounters with stars. Finally, very high velocity blobs of gas from supernova explosions that travel through very low density regions of space will escape into the low-density intergalactic space where they will expand before they ever reach the required mass and compactness necessary for them to collapse gravitationally into gas giant planets. These very high velocity blobs of gas will enrich the intergalactic medium with heavy elements produced from supernova explosions.