|M.Sc Student||Nadav Goulinski|
|Subject||Capture of free-floating planets by stellar systems|
|Department||Department of Physics||Supervisor||Dr. Ribak Erez|
|Full Thesis text|
Evidence of exoplanets with orbit that are misaligned with the spin of the host star may suggest that not all bound planets are born in the protoplanetary disk of their current planetary system. Observations have showed that Jupiter-mass objects that may be considered as free floating planets can exceed the number of stars in our galaxy, implying that capture scenarios may not be so rare. Measurements of the relative spin-orbit alignment of transiting extrasolar planets reveal that a considerable fraction of the hot Jupiters have misaligned spin-orbit, in contrary to the expectation of a close alignment between the spin of the star and the orbital motion of the planets, as they all should inherit angular momentum from the protostellar disk.
It was suggested that high-speed gas blobs from the explosive death of stars may form a free-floater by accretion of ISM ambient matter as they slow down, cool by radiation and collapse to a hot Jupiter if their mass exceeds the Jeans mass. Such filamentary structures are observed in large numbers in nearby supernova remnants, planetary nebulae, star formation regions and are considered to be common in these stellar stages. Some of these blobs were observed as dense conglomerations of gas within H II regions and where referred as globulettes. A wide-field image of the Helix Nebula in the 2.12μm molecular hydrogen line shows more than 40,000 blobs that constitute the only source of the H2 surface brightness. If this is a typical number of blobs for a planetary nebula, then there may be 1000 Jupiter mass objects for every star in the galaxy. In order to predict the capture rate of this kind of objects by planetary systems, we calculate the capture cross-section by simulating three-dimensional scattering of free-floating planet by a star-planet binary. The simulation was performed for different masses of the free-floating planet, as well as different inclination angles and velocities at which it approaches the planetary system.
We show that capture cross-section is highly sensitive to the initial velocity of the free-floater, dropping to 10% for free-floater with an initial velocity of only 1 km/s, results in a very low capture rate of one capture every 218 years in the thin-disk, in which the free-floater will most likely be loosely bound and may get easily ejected. Although captures are rare, light stars have longer lifetime and so they are more likely to capture a free-floater; since light stars are much more abundant, about ∼1% of all stars are expected to capture a free-floater during their lifetime.
Our simulation assumed point masses and only the gravitational forces that they produce. We did not include any additional effects that may subtract energy from the interaction free-floater. This means that although the capture probability is based mostly on loosely bound captures, dynamical friction and tidal effects may tighten these captures.