Since I was about the age of 10, I generally had a hard time with the "standard model" for solar system creation, or specifically the concept of core accretion. Now, with every new discovery of new solar systems, our long held concepts are sinking fast.
The long-held standard model theory goes like this:
Planets are formed from a protoplanetary disc of dust and gas that is orbiting a star. Gravity causes heavy elements such as metals and silica migrate towards the star due to their greater density, and lighter elements such as gasses to migrate towards the outer edges of the solar system.
Gravity then, over great periods of time, causes elements to condense into protoplanets, much like the dwarf planet Ceres and the asteroid Vesta, both of which are thought to possess a differential interior. As a result, rocky planets like Earth and Mars from closer to the star, gas planets like Jupiter and Saturn form farther from a star, and because all planets formed from the same disc, they all orbit their star in the same direction.
This theory perfect explains how a solar system like ours could potentially come into existence. The only problem cane in the early 1990’s when we discovered exosolar planets (planets outside of our solar system that orbit other stars), the vast majority of these solar systems contain planets as massive, (and even several times more massive), as Jupiter orbiting closer to their star than Mercury orbits to the sun. These types of planets are commonly referred to as “hot Jupiters” or “Super Jupiters.”
In addition, it also does not explain how the planet-sized terrestrial moons of the outer solar system could have formed so far away; specifically Jupiter’s Ganymede and Saturn’s Titan. Ganymede, with a diameter of 5,268 km (3270 miles), and Titan (1,575 kilometers [1,600 miles]) are both larger than the planet Mercury. Titan’s complex atmosphere and weather are also only rivaled by Earth and Venus, as far as rocky planets are concerned.
With new knowledge and understanding of moons in the outer solar system, and the discovery of dozens and dozens of exosolar planets, clearly the concept of core accretion and the standard model had to be rethought.
Then, a new explanation for all of the hot Jupiters came, but with a discouraging headline: Solar systems discovered with super Jupiters orbiting close to their star spells bad news for life as we know it.
The idea being that they these super-massive gas giants form far away from the star, as with the standard model, and then spiral inwards, ending up in tight, circular orbits extremely close to their parent star. The act of spiraling inwards is thought to pretty much destroy rocky Earth-like planets.
So, where could the theory have gone wrong?
Super Jupiters may not spell death for Earth-like planets; they could actually spell life.
Suppose that a planet ten times the mass of Jupiter orbits so close to its parent star that it does collide. Why wouldn't the explosion be great enough to hurl a large enough portion of that matter back into space to form planets made out of heavy elements? Perhaps it could.
Our Jupiter would have enough energy to cause about 100 billion tons of material to be released into space, but some of the largest exoplanets we've discovered could cause nearly one trillion tons of matter to be ejected into space.
The Earth is about 6,000 billion tons.
A few large impacts like that is enough to seed an already existing protoplanetary disc with heavier elements such as metals, primarily iron, and other silica that would allow what we refer to as terrestrial planets to be formed.
That does not take into account the amount of mass that would be shot back out into space due to the escape velocity caused by the explosion or the interactions of the star and a large magnetosphere of a gas giant.
Super-massive gas giant planets could actually accrete the metals and silica needed for an Earth-like terrestrial planet to form, bring that material close to a star, and release it in the best suited region of space for life to evolve on its own; the so-called “life zone.”
Would a Jupiter sized planet spiraling in towards a star really destroy all planets in its wake?
The answer is not necessarily.
Neither Jupiter nor Saturn destroys all of the objects in their path. Many of the objects get flung into different trajectories around the sun, and others are captured through a process known as aerocapture, and become natural satellites to the giant planets.
Along those lines, there have been Jupiter-esque planets found orbiting in "the life zone" of other stars, yet the discussion of Earth-like satellites orbiting these planets almost never comes to the surface. It is likely that life could evolve around a "blue moon" of a gas giant.
The Standard Model is not so “standard” anymore.
There is a distinct possibility that the first Earth-like planet found might not actually be a planet, but a moon orbiting a gas giant. The November 6th announcement of the discovery a fifth planet orbiting 55 Cancri that is Saturn-esque promps great excitement because this star has been watched for years, it has the most known planets outside of the sun, and the new planet is in the "life zone," thus it could have Earth-like moons surrounding it. (Universe Today has a nice write-up: Universe Today » Fifth Planet Found Orbiting 55 Cancri)
More pressingly, scientists need to work towards a new model for solar system creation, one that explains super Jupiters and distant rocky planets, and be open to the possibility that the Sol system (or solar system) once contained one or more hot Jupiters of its own.