[ExI] Silence in the sky-but why?

Alfio Puglisi alfio.puglisi at gmail.com
Mon Sep 2 20:10:12 UTC 2013

On Mon, Sep 2, 2013 at 8:07 PM, BillK <pharos at gmail.com> wrote:

> On Mon, Sep 2, 2013 at 6:35 PM, Alfio Puglisi wrote:
> <snip>
> > In the following years, all sorts of oddball systems have been found:
> some
> > have multiple Jupiters close to the star, others have high-eccentricity
> > orbits that periodically sweep a system clean of small debris like an
> > Earth-sized planet, etc. It is true that selection effects still make it
> > very difficult to detect whether a star hosts a planet similar to our
> own,
> > but we know at least that our solar system, with all the planets in
> > almost-round orbits and nicely grouped by size, cannot be taken as a
> typical
> > example.
> >
> >
> Initially extrasolar planets found were all giant planets because they
> were easier to detect.
> But the Kepler mission now has thousands of candidates.
> <https://en.wikipedia.org/wiki/Extrasolar_planet>
> Quote:
> In 2013, estimates of the number of Earth-sized planets in the Milky
> Way ranged from at least 17 billion to at least 144 billion.

Oh yes, but what kind of stars are hosting those planets, and at what
distance? Turns out that most of those stars are M-type, way cooler than
our sun, and to be habitable, the planed must be so close to the star that
it will be probably tidally-locked to present always the same face to the
red star. A very, very different environment from our one. In all, only
about 10% or less of Kepler's candidates could be considered potentially
habitable planets, even discouting the tidal lock.

All this still suffers from selection effects, because:
1) close planets on small stars are easier to find, even with Kepler
2) our idea of what an habitable planet is could well be biased by our
sample of one

For added fun, have a look at this gallery:

What I find interesting is the eccentricity: a lot of the extrasolar
planets have very high eccentricity, some even resemblng cometary orbits!
Look at how many of the orbits are off-center with respect to their star!
If you click on the green circles, representing the habitable zone, you'll
see that even most of the potentially habitable planets have orbits that
are far from circular. Again selection effects count here, because a high
eccentricity orbit is easier to find, but not by a lot. This time it seems
a real feature of planets, that our solar system is sorely missing.
Probably to our advantage, it must be said...

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