Earth-sized worlds much more common than giant planets (BBC)

[eppur_se_muova]

By Pallab Ghosh
Science correspondent, BBC News

Nearly one in four stars like the Sun could have Earth-sized planets, according to a new estimate published in the journal Science.

A US team has found that on average small, so-called rocky planets are much more common in orbit close to their star than giant planets planets similar in size to Jupiter.

This estimate is based on observations from nearby stars taken by the the twin 10-metre Keck telescopes in Hawaii. These show that 22 of the stars had detectable planets.

The researchers estimated that about 1.6% of the Sun-like stars in their sample had Jupiter-size planets and 12% had so-called "super-Earths", which are between three and 10 times the mass of the Earth.
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Based on these statistics, Dr Howard says that Nasa's Kepler space telescope - which is to to survey 156,000 stars - will detect between 120 and 260 "plausibly terrestrial worlds".
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more: http://www.bbc.co.uk/news/science-environment-11647089




And no signals from any so far ... what if it turns out that ***WE*** are the most advanced species around? I don't think we're ready for that kind of responsibility.

[Xipe Totec]

1. Maybe they're just advanced enough to turn off the porch light and hide behind the curtains nt

[qazplm]

2. most disagree with me

but I think there is at least some possibility that at least "locally" we might be the most advanced civilization within say a 100 light year bubble or roughly the time we could have possibly picked up another signal.

[exboyfil]

3. Pick up a copy of The Eerie Silence

by Paul Davies. I am a Saganite when it comes to wanting to believe that intelligent life is ubiquitous, and Davies has run a portion of SETI, so the book is a real downer. Several points to consider:

1. We need a third generation star to form life as we know it (2nd generation gives us heavier elements like carbon)
2. While bacterial life emerged very quickly, it took 4.2B years for multicellular life to evolve.
3. Our species (and the whole planet) has gone through several "keyhole" events including something really bad that happens about every 20 million years (Davies theorizes it is as the sun moves above the galactic plane our planet is swamped in radiation).
4. How long is a technological civilization extant? We know from experience that our own empires have a tendency to wind down.
5. The window for developing technological life is very narrow (we used 4.7B years of a total of about 10B for our sun).
6. As far as we can tell so far, life had only one start (all living things have a common ancestor). Darwinian clean up may explain this, but Davies point is that no universal organizing principal has been shown, and without such a principle we can't conclude anything about the existence of other life.
7. Where are they? Projections state a technological civilization could colonize the galaxy in 300,000 years. Even 10 million years is nothing compared to the age of the galaxy (see point about 3rd generation star).
8. Relativity and the light speed limit makes any civilization too far away inconsequential to the discussion. We will never interact. As a first pass we need to only consider our own galaxy.
9. The formation of multicellular life required the incorporation of mitochondria (which at the time was a form of bacteria) into a cell. A recent paper discussed how improbable such an event would be. It took our planet 4.2B years to pull it off.

Davies thinks we should be looking in other ways besides SETI. His big thing is looking for microbial life which does not share a common ancestor.

[Phoonzang]

5. I believe Davies is supporting the Benford method for detecting alien signals

...see post #4.

[AlecBGreen]

8. all good points

just one correction. Multicellular life has been here for much longer than 300 million years (actually about 1 billion years) so it took 3.5b to show up. I have some Devonian fossils that are more than 400 million years old!

[exboyfil]

10. Is Earth is 4.7B?

Evidence of multicellular life dats from about 500M (I guess I have seen some dates pushing back to 1B). Date of 4.7B could be shorter (I guess latest estimate is about 4.5B). My count was from formation of earth and not from development of bacteria (which we really don't know when that actually happened or even if it happened on earth).

However you slice it it took a really long time to develop multicellular life (on the order of 25-33% of the available time for our sun), and half the age was used in developing technological life. Our sun has some unique properties about it as well (higher than normal concentrations of heavy elements for example). If one is not too careful, you start to think anthropic principles.

[Posteritatis]

12. Multicellular life goes pretty far back

There's vertebrates going awhile past 500M; I remember hearing of fish fossils into the 530s or 540s with simpler multicellular life going back at least another hundred million or three, and there's bacterial fossils going back 2.5 Ga, with disputed finds going as far back as 3.5.

[Phoonzang]

4. Just because we haven't picked up a signal doesn't mean we're alone

in the local neighborhood. It also doesn't mean there aren't signals being transmitted. One good reason is that we do NOT have the capability to pick up radio leakage from a civilization on a technological level similar to ours. Most signals (TV, radio, etc) would fade into background noise a few light years out. Radar is one of the exceptions I believe. Anyway, if there were a technological civilization as close as Alpha Centauri, we'd be hard pressed to find them. Let's also not forget that as civilizations become more advanced, they actually become quieter. Fiber optics, spread spectrum transmission....eventually a civilization might become almost invisible.

Another reason that us not finding a signal doesn't mean much is that current SETI methods require an alien civilization to be intentionally sending a signal towards Earth. Now this means that they have to know we're here (or at least suspect we're here) and that we have to be looking at the exact patch of sky, at the exact time the signal is being sent, and listening on the exact frequency or frequencies. SETI's Seth Shostak compared the chances of this happening to two hunters in a forest firing their guns at the same time and having the bullets collide.

These reasons, combined with others that I can't think of right now because it's too early mean that the lack of a signal is no reason to get into "Humans are so unique and special" mode.

Oh, there's also a fairly recent paper by astronomer Gregory Benford that puts forth the theory that there are easily detectable signals but we're looking in the wrong way and in the wrong area of space. He postulates that most advanced civilizations would be a couple of thousand light years away towards the core of the galaxy.

http://www.physorg.com/news198835228.html

[exboyfil]

6. Towards the core

Close to other stars - other stars who like to do wild things like Gamma Ray Bursts, disrupt orbits of out lying objects, novas, etc?

I don't think so. I think being in a rural area is better for your health.

We should look for weird radiation signatures (like what would come from a Dyson Sphere) or interstellar propulsion. Spill over from 20th century radio/television won't be detectable over about 100 light years.

I agree that the lack of signals does not lead to the conclusion of being alone, but looking at the development of technological life from the ground up is very instructive.

[Phoonzang]

7. True, but I didn't mean in the core. Just directionally corewards.

I don't remember exactly how far corewards, but I think it was 1-2,000 light years. Still within the supposed "Galactic Habitable Zone."

[pokerfan]

9. about 5% of the stars in our galaxy are G-type stars

source: http://astrobio.terc.edu/drake/approp_stars.htm

100 billion stars in the Milky Way (conservative end of the estimated range according to wiki)

so 1/4 of 5% of 100e9 = 1.25 billion "Earths" in the galaxy

[Posteritatis]

11. And that doesn't include Earth-sized planets in closer orbit around smaller stars

Dwarf stars would be immensely stable environments, stuff like that.

[pokerfan]

13. That's a good point

For example, there appear to be twice as many K-type stars (orange, longer lived) as G-types. That would triple the estimation from 1e9 to 3e9. I was just keeping it conservative (AKA pessimistic) and demonstrating how many potential habitable planets there still might be. Taking the upper range (400B) for the number of the stars in the milky way quadruples that number to 12e9. Bit by bit, we are piecing together the parameters of the http://en.wikipedia.org/wiki/Drake_equation">Drake equation: