So, the range of possibilities of what could be
a habitable planet is probably much larger.
Then we've imagined up to now.
What are the lessons we've learned so far from our study of extra solar planets
is that what's out there is much richer than we've imagined.
We thought all solar systems would be like our own.
And what we're discovering is that there are a range of possible planets
that are a much larger range than we see in our own solar system.
When we think about planetary properties, we can think about
a planet's distance from the Sun and a planet's mass as the two minimal things.
That determine its properties, but even given its distance from the sun and
its mass, different planets will have different compositions.
Some planets might be dense like Mercury, and from
the heavy materials that can survive in the inner portions of the solar system.
While other planets might have formed.
In the outer regions of the solar system, so it might be ice-rich or
even hydrogen-rich and then migrate in.