Our bizarre solar system has no hot Jupiter. XO-7 b is a hot Jupiter which orbits its star once roughly every 3 days (1). Of all the solar systems in the heavens ours seems to be most unusual. Our solar system has orderly circular orbits, with rocky planets near the Sun, and gassy icy planets farther out. That order and stability seems to be the exception not the rule. Humanity has now observed numerous solar systems and found one type to be very common. A solar system which contains a hot Jupiter like planet orbiting much closer to the star than Mercury does to the Sun.
XO7-b is interesting for being a hot Jupiter orbiting a “sun-like” G-Type star at a distance far closer than Mercury orbits our own Sun*. This has been taken as a sign that such a solar system will not likely contain and Earth like planet. However, this one also has a planet with a mass in the range four times that of Jupiter, and orbiting with a period of at least three to four years (1). Thus, orbiting approximately where the asteroid belt would be in our solar system. To understand why this is interesting, a brief review of the process of star and planetary system formation is in order. This model is based on our own solar system and on what we see of other solar systems.
A Crash Course In Solar System Formation
1.) A cloud of gas and dust begins to collapse under its own gravity spinning faster as it becomes more compact.
2.) As the disk flattens the central dense region forms a star which eventually begins to fuse hydrogen to helium. This releases a lot of heat. This causes more volatile compounds to vaporize and migrate further away from the star.
3.) At the same time smaller dense regions in the disk form planetoids. (For an example of such an object in the current solar system see Ceres).
4.) Sine volatile gasses and ices only survive further out ices and gasses accumulate to form jovian planets in the outer solar system.
5.) Rocky planets form nearer to the star in the inner solar system.
This is where many other solar systems begin to diverge from our own solar system. The precise history of each solar system and the planets they contain depends on their history of orbital interactions and collisions. In the case of a great many solar systems, one planet, of Jupiter mass or greater, will spiral inwards towards their host star. In the process any rocky planets, like the Earth or Venus would be ejected from the star system. This is what computer simulations have shown (2).
We have also observed that when hot Jupiters do this they tend to simply wreck the rest of the planetary system. Their gravitational interaction with their host star is so dominant that it may just obscure the signatures of Earth mass planets or they truly do eject almost every other planet.
Possibilities This Observation Opens Up.
Star system XO7 shows us that it is possible to have both a hot Jupiter and a massive Jupiter like body in a more Jupiter like orbit, around a sun-like star. This is reason to not give up hope for an Earth mass planet or moon existing in such a solar system. To resolve these questions beyond computer modeling and theoretical guessing will take ever better angular resolution images.
You may ask, is there some fundamental reason why a solar system like ours should be rare . For this we can only speculate. One possibility is the anthropic principle. It states that if the physical conditions were not right, we wouldn’t be here. Intelligent life appears to be rare therefore the environment it needs should be rare. That is, the reason our solar system looks the way it does. Another is that perhaps solar systems obey some version of the laws of thermodynamics and seek a stable state of unchanging entropy. With the most stable state being the most massive planet orbiting closest to the star, and a meta stable state being the state of our own solar system. Maybe it is just random chance, combined with the fact that hot Jupiter systems are easiest for us to study right now? If you could only see blue and green light it wouldn’t mean the red light does not exist.
This last reason is why we always need ever bigger telescopes which can concentrate more light and bring out more details. Maybe these hot Jupiters don’t necessarily destroy Earth mass planets as they drive closer to their stars. Maybe they throw them out of the orbital plane but not all the way out of the planetary system?
We will only truly know if we look.
Striving, in various ways, spritual, artistic, and especially scientific, to know how things really work is what makes us humans.
Next week I intend to write about a theoretical type of telescope which would not need to be on top of a mountain, but which would use our Sun’s gravitational mass to gather and focus light. The catch, we won’t really be able to steer it.
1. XO-7 b: A transiting hot Jupiter with a massive companion on a wide orbit. Crouzet, Nicholas, et al. s.l. : arXiv, 2019, Arxiv E-Print (Accepted for Publication in the Astronomical Journal. .
2. Gas Disks to Gas Giants: Simulating the Birth of Planetary Systems. Edward W. Thommes, Soko Matsumura, Frederic A. Rasio. August 8, s.l. : Science, 2008.