Exoplanet orbital and physical parameters

Most known extrasolar planet candidates have been discovered using indirect methods and therefore only some of their physical and orbital parameters can be determined.

Gravitational instability models might produce planets at multi-hundred AU separations but this would require unusually large disks.

Some planets with larger orbits have been discovered by direct imaging but there is a middle range of distances, roughly equivalent to the Solar System's gas giant region, which is largely unexplored.

[17] For weak Doppler signals near the limits of the current detection ability, the eccentricity becomes poorly constrained and biased towards higher values.

It is suggested that some of the high eccentricities reported for low-mass exoplanets may be overestimates, because simulations show that many observations are also consistent with two planets on circular orbits.

[19] Radial velocity surveys found exoplanet orbits beyond 0.1 AU to be eccentric, particularly for large planets.

Transit data obtained by the Kepler spacecraft, is consistent with the RV surveys and also revealed that smaller planets tend to have less eccentric orbits.

[9] However, a combination of astrometric and radial-velocity measurements has shown that some planetary systems contain planets whose orbital planes are significantly tilted relative to each other.

[22] More than half of hot Jupiters have orbital planes substantially misaligned with their parent star's rotation.

For close-in exoplanets, the general relativistic contribution to the precession is also significant and can be orders of magnitude larger than the same effect for Mercury.

The quadrupole mass moment and the proper angular momentum of the star are 1900 and 400 times, respectively, larger than those of the Sun.

The last few giant impacts during planetary formation tend to be the main determiner of a terrestrial planet's rotation rate.

During the giant impact stage, the thickness of a protoplanetary disk is far larger than the size of planetary embryos so collisions are equally likely to come from any direction in three-dimensions.

This results in the axial tilt of accreted planets ranging from 0 to 180 degrees with any direction as likely as any other with both prograde and retrograde spins equally probable.

[30] Gravitational tides tend to reduce the axial tilt to zero but over a longer timescale than the rotation rate reaches equilibrium.

However, the presence of multiple planets in a system can cause axial tilt to be captured in a resonance called a Cassini state.

Hot Jupiters are likely to evolve towards zero axial tilt even if they had been in a Cassini state during planetary migration when they were further from their star.

Hot Jupiters' orbits will become more circular over time, however the presence of other planets in the system on eccentric orbits, even ones as small as Earth and as far away as the habitable zone, can continue to maintain the eccentricity of the Hot Jupiter so that the length of time for tidal circularization can be billions instead of millions of years.

Also, astrometric observations and dynamical considerations in multiple-planet systems can sometimes provide an upper limit to the planet's true mass.

In 2013 it was proposed that the mass of a transiting exoplanet can also be determined from the transmission spectrum of its atmosphere, as it can be used to constrain independently the atmospheric composition, temperature, pressure, and scale height,[36] however a 2017 study found that the transmission spectrum cannot unambiguously determine the mass.

[38] Prior to recent results from the Kepler space observatory, most confirmed planets were gas giants comparable in size to Jupiter or larger because they are most easily detected.

For colder gas planets, there is a maximum radius which is slightly larger than Jupiter which occurs when the mass reaches a few Jupiter-masses.

[39][40][41] Even when taking heat from the star into account, many transiting exoplanets are much larger than expected given their mass, meaning that they have surprisingly low density.

Besides the inflated hot Jupiters, there is another type of low-density planet: super-puffs with masses only a few times Earth's but with radii larger than Neptune.

[46] Thermal evolutionary atmosphere models suggest a radius of 1.75 times that of Earth as a dividing line between rocky and gaseous planets.

[48] An independent reanalysis of the data suggests that there are no such dividing lines and that there is a continuum of planet formation between 1 and 4 Earth radii and no reason to suspect that the amount of solid material in a protoplanetary disk determines whether super-Earths or mini-Neptunes form.

[49] Studies done in 2016 based on over 300 planets suggest that most objects over approximately two Earth masses collect significant hydrogen–helium envelopes, meaning rocky super-Earths may be rare.

They orbit very close to their stars, so they could each be the remnant core (chthonian planet) of an evaporated gas giant or brown dwarf.

If a remnant core is massive enough it could remain in such a state for billions of years despite having lost the atmospheric mass.

Also, these stars have high UV radiation and winds that could photoevaporate the gas in the disk, leaving just the heavy elements.