Title: Jupiter’s Decisive Role t’sn the Inner Solar System’s Early Evolution
Authors: Konstantin Batygin and Gregory Laughlin
First Author Institution: Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
Status: Submitted to Proceedings of the National Academy of Sciences of the United States of America
Since the discovery of the first extra-solar planet around another star in 1995 we now know of ~ 4000 candidate planets in our galaxy. All of these discoveries have been key in improving our understanding of the formation of both planets and star systems as a whole. However a full explanation of the processes which form star systems is still elusive, including a description of the formation of our very own Solar System.
The problem with all these exoplanets and star systems we’ve discovered so far is that they suggest that the Solar System is just weird. Most other systems seem to have massive planets similar to the size and mass of Neptune but which are the distance of Mercury to the Sun from their own star (often these planets can be as big as Jupiter – since these are the easiest for us to detect). For example the famous Kepler-11 system is extremely compact, with 6 planets with a total mass of about 40 Earth masses all within 0.5 AU (astronomical unit – the distance of the Earth from the Sun) orbiting around a G-type star not at all dissimilar from the Sun.
Figure 1 shows all the Kepler detected planets with masses less than Jupiter within the orbit of Mars from their own star. So if most other star systems seem to be planet mass heavy close into their star – why is the Solar System so mass poor and the Sun so alone in the centre?
The authors of this paper use simulations of how the orbital parameters of different objects in systems change due to the influence of other objects, to test the idea that Jupiter could have migrated inwards from the initial place it formed to somewhere between the orbits of Mars and Earth (~ 1.5 AU). The formation of Saturn, during Jupiter’s migration, is thought to have had a massive gravitational influence on Jupiter and consequently pulled it back out to its present day position.
If we think first about how star systems form, the most popular theory is the core-accretion theory, where material around a star condenses into a protoplanetary disc from which planets form from the bottom up. Small grains of dust collide and stick together forming small rocks, then in turn planetesimals and so on until a planet sized mass is formed. So we can imagine Jupiter encountering an army of planetesimals as it migrated inwards. The gravitational effects, perturbations and resonances between the orbits of the planetesimals and Jupiter ultimately work to cause the planetesimals to migrate inwards towards the Sun. The simulations in this paper show that with some simplifying assumptions the total amount of mass that could be swept up and inwards towards the Sun by Jupiter is ~10-20 Earth masses.
Not only are the orbital periods of these planetesimals affected but their orbital eccentricities (how far from circular the orbit is) are also increased. This means that within that army of planetesimals there’s now alot more occasions where two orbits might cross initiating the inevitable cascade of collisions which grind down each planetesimal into smaller and smaller chunks over time. Figure 2 shows how the simulations predict this for planetesimals as Jupiter migrates inwards.
Given the large impact frequency expected in a rather old protoplanetary disc where Jupiter and Saturn have already formed, the simulations suggest that a large fraction, if not all, of the planetesimals affected by Jupiter will quickly fall inwards to the Sun, especially after Jupiter reverses its migration direction. This decay in the orbits is shown in Figure 3 with each planetesimal getting steadily closer to the Sun until they are consumed by it.
The orbital wanderings of Jupiter inferred from these simulations might explain the lack of present-day high mass planets close to the Sun. The planetesimals that survived the collisions and inwards migration may have been few and far between, only being able to coalesce to form smaller rocky planets like Earth.
The next step for this theory is to test it, on another star system similar to our own with giant planets with orbital periods exceeding 100 days. However our catalogue of exoplanets is not complete enough to provide such a test yet. Finding these large planets at such large radii from their star is difficult because their long orbital periods coincide with how often we have the chance of observing a transit. For example if we wanted to detect a Neptune-like planet in a Neptune-like orbit, a transit would only occur every 165 years. Also, detecting small planets close to a star is also difficult as the current telescope sensitivities don’t allow us to detect the change in the light of a star for planets so small.
So perhaps we just haven’t been looking long enough or with good enough equipment to find star systems like ours. However with missions like GAIA, TESS and K2 in the near future perhaps we’ll find that the Solar System is maybe not as unique as we think.