Scientists Estimate At Least 17 Billion Earth-Size Planets Inhabit Milky Way | Hacker News Reader

Scientists Estimate At Least 17 Billion Earth-Size Planets Inhabit Milky Way

space.com

53 pointsG5ANDY48 comments

This is sort of terrifying, given that we haven't observed any space-faring species from any of those other Earth-sized planets yet.

http://en.wikipedia.org/wiki/Great_Filter

Maybe we are who we are looking for:

http://news.ycombinator.com/item?id=4843540

I don't worry about that too much.

Consider a community of people with little technology that live away from any major trade routes, and thus don't see ships or planes. They would be oblivious to all the internet, TV, radio and postal traffic going on around them despite its huge volume. Now, I can't think of any such islands that are totally out of contact in 2013, but there are still uncontacted tribes in the Amazon basic (that we know about) and probably a few in other places (that we don't know about).

Since interstellar travel would require something like a warp drive to be any way practical, and we have yet to crack that problem, it seems quite likely to me that we're also incapable of observing traffic of that kind going on in our (galactic) vicinity. I don't see any particular reason that more advanced civilizations would go out of their way to reveal themselves to us; indeed, it bothers me that most alien invasion movies/novels tend to skip over the question of what is so special about Earth that aliens should cross vast distances to plunder our resources in particular when it would be so much easier to just grab a passing asteroid or comet.

Why, to collect us for their zoos of course.

We may also make good pets, like the monkey on the shoulder of some alien Michael Jackson.

"Now, I can't think of any such islands that are totally out of contact in 2013"

We can always count on Cracked to point the way. ;-)

http://www.cracked.com/article_19976_6-isolated-groups-who-h...

Human recorded history is what 5-10k years? Pretty tiny in comparison to the 4 billion years earth has been around. We might've had visitors already!

I'm in awe of how much we've discovered about the universe in the last decade of science exploration.

The space programs Cassini, Kepler and Curiosity, as well as earth bound programs like VLT and LHC really have expanded our understanding of the laws of nature to levels I didn't think I'd see in my lifetime.

We live in fascinating times.

There's always the problem of when.

While it's becoming more likely that a life supporting planet exists outside our solar system, we have the problem that mankind's recorded history is measured in thousands of years (and more than likely will only be measured in thousands of years) while the universe is a tale of billions of years.

I'd still say we are likely to see artificial satellites at some point, as our own would likely survive for millions of years in some capacity (even as junk), but the possibility of direct communication is probably never going to happen.

I'm curious how many Earth-sized planets have a twin planet like our moon. Without the moon we'd be tidally locked, and despite all our water we wouldn't be nearly as life-friendly.

Mercury and Venus aren't tidally locked.

They're quite close to it. Mercury takes two of its own years to have one day, and Venus's year is less than two of its days.

This is generally suspected to be because of large primordial impacts, much like the one which it is proposed created our moon. I don't think there's anything intrinsic about Goldilocks zone orbiting rocky planets which would lend to tidal-locking with the star.

It's not their distance from the sun, it's the presence of liquid being affected by gravity and creating a tidal bulge. The bulge slows down the planet's rotation via friction, and eventually the planet stops rotating.

Mercury is in a more elliptical orbit; its 'tidally locked' at the perihelion but when its further away it gets a half-turn in. So each side spends half a year baked, half a year frozen.

Having Jupiter nearby to suck up a lot of stray flotsam and jetsam is quite helpful, too. Much harder to make life while under a constant rain of interplanetary debris.

And also how many of these are in the Goldilocks zone.

I'd be curious too. For me the Fermi paradox is best satisfied by the 'Rare Earth' hypothesis. There are just too many infinitely rare combinations of variables required to support the kind of life we see on our planet.

I don't hold out any hope of finding much beyond the odd microbe.

Is having a large moon a requirement for not being tidally locked?

My understanding was that planets further from the star would take more time to get tidally locked. Since we are quite far from the sun, it would take some time even without a moon.

You're correct, it isn't. Planets in very close orbits tend to be tidally locked. Instead the significance of the moon is thought to be as a stabilizer for the Earth's rotational axis. However, our axis is not exactly as ultra stable as some people think - it's still shifting up to a few degrees every few million years.

It's also worth noting that six out of nine planets in this system are oriented in the same way as Earth and none of them has the same moon configuration as us. For example, Mars has pretty much the same tilt as us and its moons are pitiful.

I personally don't believe it's a requirement for life. We simply lack the empirical data needed to make any assumptions about habitability or even the basic configuration of habitable star systems. Until then, looking towards twin Earths is probably a good start to look for life but it's certainly not the only kind of environment stable enough to support it.

Fascinating. Our little rock could be "one in a billion" and we would still have 16 other civilizations to meet. Just in our galaxy.

Actually we would expect to have 17 other civilizations to meet. The events are independent, so observing our own civilization doesn't affect the odds for the rest of the galaxy.

But we're not interested in Earth, so we'd expect to see 16.999999999 other civilizations.

Yes, but I think you may have a few too many sig figs there.

So does 17 :-)

I guess the number of sig figs when you write out "a billion" is ambiguous.

I am curious How did you get to this number ?

If there are 17 billion Earth sized planets out there and we're "one in a billion" that would mean there are 17 planets as perfect as ours that there is life. 17 - 1 = 16. The billion figure was just pulled out of thin air, "one in a X" is a common expression and for this purpose billion seemed appropriate. If we're actually only one in a million, we'll be busy meeting our new friends.

>If we're actually only one in a million, we'll be busy meeting our new friends.

The Milky Way is roughly a disk, so let's think in two dimensions, and for the sake of being reasonable, let's just talk orders of magnitude.

The area of the Milky Way disc is on the order of 10^9 square light years. If there are 10^4 civilizations in the Milky Way, that means we should expect one other civilization within a 10^5 square light year neighborhood, meaning that we would expect the nearest civilization to be on the order of 100 light years from us.

Over that distance, it would be plausible for two civilizations to transmit information to each other, but "conversations" would take generations. And that's if we ever managed to find them.

Indeed, "one in a billion" might mean that our civilization is the civilization one among among a billion life supporting planets.

How does civilization last after it acquires the ability to transmit and receive radio waves? Human certainly seems to be at least threatening portions of earth ecology. Suppose alien ecologies are, on average, more fragile and easily disrupted by the unintended consequences of transformation. Civilizations might exist but be a quite transient phenomena over the galaxy. It is very hard to predict, naturally but interesting to speculate.

One in a billion, though, what are the odds of life existing? It might be much higher than that. I really don't know much in this area of science though.

Regarding the existence of life...

Seems all that's really required to kickstart life and evolution is any self-replicating structure.

Chemicals are well suited for creating complex structures, but it doesn't rule out self-replicating forms embedded in other kinds of matter held together with other kinds of force.

This is interesting, but I think an estimate of the number of planets with lots of liquid water on the surface would be more exciting. Liquid water is key to life as we know it, so knowing how common it is in the galaxy might give us some idea how likely we are to find extraterrestrial life.

There's a lot more to life than just liquid water. In fact there are a huge array of requirements including distance from the sun, right arrangement of planets, tidal systems, plate tectonics...

I remember an old Chaos Manor column that talked about a book detailing what was needed, but I have not been able to find the book or remember the title.

Perhaps "Rare Earth" by Peter Ward and Donald Brownlee.

http://www.amazon.com/Rare-Earth-Complex-Uncommon-Universe/d...

nice book, but it was much, much older - mentioned in a column in the 80's

Why there are so many constraints on life listed? Is it to make more us special? Some people follow a different approach and say the purpose of life is to hydrogenate carbon dioxide.

http://blogs.discovermagazine.com/cosmicvariance/2010/03/10/...

I very much like the idea that life might be a consequence of entropy and emerges everywhere where energy is in flux and matter poses a threshold against the arrow of time.

It's a nice idea, but I find the 'Rare Earth' hypothesis convincing: http://en.wikipedia.org/wiki/Rare_Earth_hypothesis#Rare_Eart...

i think it's the opposite of that. by all evidence, life on Earth began as soon as it was possible, and that's just this particular kind of life with this set of particular elements under these particular constraints. there may be other chemical/physical configurations under which life is even easier, etc

Just look at the difference an active core makes by observing the difference between Earth and Mars. How likely is it that a planet has all the right things: liquid water, enough gravity (mass), active core, planets further out sweeping up the debris so it doesn't destroy said planet over and over, tidally locked moon, etc.

My guess is that Earth is very, very unique.

yea but that doesn't mean that Earth's configurations are the only ones that yield life. example: the role water plays here on earth might not even be necessary in other chemical/physical systems, or could be played by other chemicals given different gravitational/atmospheric pressures, etc

and i don't think there is anything about earth that is unsusual as far as planetary systems go

I didn't think there was anything special about the size of the earth (and apparently I'm right). Can anybody explain why this is important, I don't think the article did a very good job of that.

People are probably interested because one of the factors in the Drake equation is the number of Earth-like planets per star.

Too small, and the planet won't be able to retain an atmosphere (e.g. Mars). Too large, and it'll probably retain too much.

I have always been under the impression that the lack of atmosphere on mars was due to its magnetic field weakening and allowing solar winds to carry away the atmosphere over the billions of years.

Regarding the Fermi paradox: one of the civilization has to be the most advanced and has to be so intelligent that it obviously knows the Fermi paradox ("probably" under another "name" ; ) all too well.

Must s*ck to be that one civilization: being right about the universe actually holding several carbon-based lifeforms and yet scratching their heads thinking "We can't be the most advanced, we can't be the most advanced".

: )

The universe is relatively young in the sense that metal-rich solar systems are probably a requirement for advanced lifeforms and we haven't had that many generations of stars yet. Our sun is a "Population I" star (counter-intuitively, astrophysicists are counting backwards here) that has taken heavy elements as building material for the whole solar system from the remnants of earlier stars.

However, the Milky Way is at the same time certainly old enough to suggest we shouldn't be the first. Evolutionary timescales probably depend a lot on the environment, but even if we're taking the development of our biosphere as one of the fast examples, there should still be other comparable planets billions of years younger than ours lending credibility to the assumption that there should have been civilizations before us.

One of the big questions is also how long civilizations stay in the "compatible" phase on average. If they disappear (either going extinct or transcending into another stage) fast enough, finding and communicating with them in time will be very difficult. Like I said, extinction isn't necessarily the driving force. For example, our own sci-fi stories are absolutely rife with slightly advanced civilizations that very likely would never communicate with us.

Another favorite explanation of mine is that there could be some unknown local issue in our stellar neighborhood. Candidates scenarios for this are myriad, and they fall into two categories: there isn't anyone nearby, or we are artificially isolated somehow.

Yeah, it's not like some alien benefactors would have placed a geodesic array of transmitters in the Oort cloud that would cancel out all our inane radio spectrum emissions.

Or could they? Hmmm...

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