Another good article by RTB’s Hugh Ross
For more than a decade, astronomers have recognized that the solar system is highly unusual. It possesses cold Jupiters closer to the Sun than 14 times Earth’s distance from the Sun but lacks one or more super-Earth planets, hot Jupiters, or cold Jupiter planets with orbital eccentricities greater than 0.09. Each one of these four features, if absent in a planetary system, rules out the possibility of any kind of advanced life existing in the system.
Is there any back up to this statement?
I agree with his first statement: that the the solar system seems to not be a typical planetary system.
The rest of his statements are too strong for my taste, and one of them is wrong. These are his claims:
These four properties are necessary for the possibility of advanced life in a planetary system:
- Possesses a cold Jupiter closer than 14 AU
- Lacks one or more super-Earths
- Lacks hot Jupiters
- Lacks cold Jupiters with large orbital eccentricities
The main “back up” to these statements are dynamical evolution: for example, eccentric large planets can produce orbital resonances that cause it to either eject or collide with potentially life-bearing rocky planets.
The claim 1) I have never heard of, but I am not a planetary astronomer, so perhaps this is true.
The claim 2) is wrong. I don’t understand why he said that lacking a super-Earth is necessary for life. Super-Earths span a mass range of >1-~15 Earth masses, and planets in this mass range are extremely varied. Indeed, rocky super-Earths of ~2 Earth masses could be prime candidates for habitable worlds.
The claim 3) is probably true, as simulations showed that while forming rocky planets in a system with a hot Jupiter is possible, the rocky planet ends up orbiting too close to the star. Note however that there are many variables at play in these simulations, so perhaps a habitable planet can be formed by fine-tuning the system.
The claim 4) is statistically true. Highly eccentric cold Jupiters are known to kick out rocky planets from the habitable zone through dynamical encounters. However, this is not true for all systems. To wit, the wikipedia page for eccentric Jupiters listed some cold Jupiter planets that still allow for stably orbiting rocky planets in the habitable zone.
Edit: For claim 1): I have heard some theories that Jupiter protected the Earth by diverting spacerocks away from it through gravitational scattering. Perhaps this is what he meant, but I’m not sure.
PdotdQ’s post is a prime example of how having so many scientists/scholars from so many academic fields makes this forum such an interesting and educational place. Excellent post. (And having someone with further “inside knowledge”, like Dr. Roberts, can tell us where to find more details from scholars elsewhere.)
From Jeff Zweerink:
It’s probably best to get a response directly from Hugh. I would be playing middle-man. For example, I know that Hugh has reasons for thinking that super-earths are uninhabitable, but I don’t know the reasoning behind his conclusion.
From Hugh Ross:
For a complete answer with documentation see Improbable Planet. Very briefly, planetary systems that contain only very distant cold Jupiters will have high populations of asteroids and comets. Here, the cold Jupiters will offer little gravitational shielding for the advanced life supportable planet from collision events. Relatively nearby cold Jupiters with high orbital eccentricities will gravitational disturb the orbit of an advanced life supportable planet. Likewise, the nearby presence of hot Jupiters and superEarths will disturb the orbit of an advanced life supportable planet. Furthermore, the origin of hot Jupiters is migration from beyond the ice line. That migration will disturb the orbit of an advanced life supportable planet. It is likely, though not yet proven, that superEarths also have a migration origin. It only takes a tiny disturbance of the orbit of an advanced life supportable planet to cause problems. For an example see: https://www.reasons.org/explore/blogs/todays-new-reason-to-believe/read/todays-new-reason-to-believe/2018/07/30/exoplanets-climate-instabilities-reveal-earth-s-fine-tuning
What do you think @PdotdQ?
It is also worth remembering the number of stars we are talking about. If our solar system were 1 in a million there would still be 100,000 solar systems like ours in the Milky Way, and 10,000 trillion solar systems like ours in the larger universe.
It seems that my guess for why he claimed that one would need a Jupiter closer than 14 AU for advanced life:
is correct. My position does not change. I think his statement that
is too strong for my tastes. He seems to think this also, as he uses the words “…likely, though not yet proven…”, which is weaker than “…rules out the possibility of any kind…”. He did not address that low-mass super-Earths are prime candidates for habitable planets, so one of his claims is still off.
Help me understand this.
Super-Earths are just planets with masses more than Earth but less than Neptune. This is a very poor classification, as the planets in this range span rocky worlds (low-mass super Earths) to very gaseous planets closer in properties to Neptune than to Earth.
Rocky, low-mass super Earths can be host to life. Indeed, from wikipedia:
According to one hypothesis, super-Earths of about two Earth masses may be conducive to life. The higher surface gravity would lead to a thicker atmosphere, increased surface erosion and hence a flatter topography. The end result could be an “archipelago planet” of shallow oceans dotted with island chains ideally suited for biodiversity. A more massive planet of two Earth masses would also retain more heat within its interior from its initial formation much longer, sustaining plate tectonics (which is vital for regulating the carbon cycle and hence the climate) for longer. The thicker atmosphere and stronger magnetic field would also shield life on the surface against harmful cosmic rays.