[ExI] Silence in the sky-but why?

Alfio Puglisi alfio.puglisi at gmail.com
Mon Sep 2 17:35:26 UTC 2013

On Tue, Aug 27, 2013 at 7:27 PM, BillK <pharos at gmail.com> wrote:

> On Tue, Aug 27, 2013 at 5:41 PM, Adrian Tymes wrote:
> <snip>
> > If the odds of intelligent, tool-using life with the potential for space
> > travel arising around a star, in a given five billion years of the star's
> > lifespan, is somewhere around one trillionth (readily justifiable when
> you
> > multiply together all the factors it would need to overcome, even before
> the
> > species gains the capacity to wipe itself out), then we would indeed
> expect
> > there to be exactly one intelligent species by now - and here we are.
>  Not
> > counting anything we create, we could expect on the order of another ten
> > billion years before another species like us came along.
> >
> >
> Gadzooks! You mean we are the only intelligent life in the galaxy!
> Now that's really frightening.

Do you know how the discovery of the first extrasolar planet went? It's an
interesting tale

[editorial note: I am writing this based on an interview on a printed
astronomy magazine many years ago, so details like the star's name and the
astronomer affiliations might be wrong. And I am unable to find references
to it on any web page. But the gist of the story should hold anyway]

During the 1990s, a team of Californian astronomers was collecting Doppler
data of hundreds of stars. The idea was that, should a planet orbit the
star, the planet's gravity will induce a small sinusoidal motion of the
star that can be detected by ground telescopes. The astronomers did some
preliminary calculation, and discovered that a small Earth-sized planet
would be way too small to induce a detectable movement. Because of the
limited sensitivity of their instruments, only a gas giant like Jupiter
could move the star about with enough force to be seen. And Jupiter-sized
planet have orbits many years long (obvious, no? like the ones in our solar
system) because their orbit need to be nice and wide in order to intersect
a lot of gas and collect all of it to reach their size (again obvious, no?).

So this was a long-term project. Collect data, store it, and have a look at
it a dozen years later when something resembling an orbit could be seen.

In 1994, another group of Swiss astronomers was looking at star called 51
Pegasi. This star was included in the Californian catalogue, but the data
had not been analyzed yet (they would have to wait some more years,
anyway). After only some days of observation, it became obvious that the
star was moving back and forth exactly as if a planet was orbiting it. And
it was so clear, because it was a Jupiter-sized planet with an orbital
period of just four days! After some months of disbelieving their own eyes,
they published the discovery of the first extrasolar planet on Nature.

Upon hearing the news, the Californians rushed to their computer, dusted
off the storage disks, and in about 30 minutes they found the same planet
in their data. Alas, they arrived too late.

They had a consolation prize, for they were following not just one star,
but about a hundred. They found two or three short-period Jupiters just by
looking, and again two or three more with some additional effort. But they
missed chance to be the first because of their preconceptions of how a
solar system should be made.

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.

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