APOD: Habitable Worlds (2014 Mar 03)

[Anthony Barreiro]

... Science simply states what it currently knows to be true, and will clarify that in as much detail as we wish to take in. But it won't go beyond that; speculation is not part of science and if science is to maintain its credibility, it must maintain that sense of propriety.

...

But an essential step in the scientific method is generating a hypothesis, your best guess about how things are, how things work, why things are the way they are. Then you come up with a way to test your hypothesis to decide if it's true. If you find support for your hypothesis, you (and the peer reviewers and readers of your scholarly publications) try to come up with other possible explanations that could explain your findings, and then you do more testing to disprove the alternative explanations and to garner more support for your favored hypothesis. So speculation is a necessary part of science. You just want to be clear about what is speculation, how much support there is for any particular speculation, and what other speculations could also be true.

[BDanielMayfield]

... Science always seems to start with a very conservative view of things. Then it gets closer and closer to the truth....and a looooong time in-between....

I don't think that's right. Science doesn't take a conservative view of things, it has no view -- at least, not if "view" means having an opinion. Science simply states what it currently knows to be true, and will clarify that in as much detail as we wish to take in. But it won't go beyond that; speculation is not part of science and if science is to maintain its credibility, it must maintain that sense of propriety.

Of course it takes a long time (usually) to get closer to the truth. That's a good thing. The "truth" as it relates to our view of reality, is something that must be determined carefully, with considerable double-checking and fine-tuning, before it can be adequately communicated to anyone other than other scientists. Scientists walk a fine line. Too many public mistakes and credibility is lost. Endless careful checking and rigor mortis sets in.

I'm sure very few scientists thought, for example, that the Earth was unique and that we wouldn't find lots of planets around virtually every star we look at. But they couldn't say much about what they as individuals thought until they as scientists began to collect facts. Now they're doing that, and now (as both you and I and most sensible people expected) it looks like planets are ubiquitous.

Rob

I like the overall import of your comment Rob, and you express things the way science should operate in principle. However, when you consider the new branch of the sciences called Exobiology there doesn't seem to be a "conservative view of things."

Bruce

[Anthony Barreiro]

According to at least some definitions of the circumstellar habitable zone, Venus, Earth, Mars, and Ceres all reside in the Sun's habitable zone. So while the recently confirmed Kepler findings are interesting and encouraging, I don't think we're quite ready to send the Welcome Wagon to our new neighbors.

[Chris Peterson]

I like the overall import of your comment Rob, and you express things the way science should operate in principle. However, when you consider the new branch of the sciences called Exobiology there doesn't seem to be a "conservative view of things."

I disagree. The publications of exobiology (the majority of which come from biologists) are as solid and robust as any other, and are typically cautious in advancing hypotheses that are hard to justify.

[BDanielMayfield]

I like the overall import of your comment Rob, and you express things the way science should operate in principle. However, when you consider the new branch of the sciences called Exobiology there doesn't seem to be a "conservative view of things."

I disagree. The publications of exobiology (the majority of which come from biologists) are as solid and robust as any other, and are typically cautious in advancing hypotheses that are hard to justify.

I'm somewhat reassured by that. I guess some of the difference in viewpoint comes from what we read. Your reading the actual science publications, whereas I'm seeing reports and discussions in pop press, websites, blogs, etc. I see too much quick acceptance of rampant speculation.

Bruce

[mollwollfumble]

> How accurate are the exoplanet sizes?
Sizes from Kepler data are based on the time that the light curve dips from maximum to minimum brightness, with a correction based on how far the planet is out of alignment with the equator, which in turn comes from the total transit time. From what I've seen, exoplanet sizes are can be in error by up to about 20%, but I'd guess 5% error is more typical.

Oops, I gave a very bad answer there. The ratio of planet radius to star radius is calculated from the square root of the dip in luminosity. The error tends to be of the order 1 to 5%. But this is swamped by the much larger error in the radius of the parent star. Main sequence stars vary in radius with age and core composition and even with the best spectrographic data it is impossible to determine the radius of the star closer than about 10 to 15%. So the radius of the planet from transit data is known to no better than 10 to 15%.

Again, elliptical orbits lead to much large errors than that.

[Florian Kainz]

In the picture, the planets are shows with different surface colors - some more blue others more yellow - and with different surface textures. Are those details based on measurable data, or are they entirely made up?

[Boomer12k]

None of them seem to be very hospitable....life, they may contain....but enjoy it not, we would.....

:---[===] *

[BMAONE23]

In the picture, the planets are shows with different surface colors - some more blue others more yellow - and with different surface textures. Are those details based on measurable data, or are they entirely made up?

All are artistic visualizations. We've not been able to image the surface of any planet outside our own solar system. We cant even see Pluto from here with any reasonabile clarity

[Anthony Barreiro]

According to at least some definitions of the circumstellar habitable zone, Venus, Earth, Mars, and Ceres all reside in the Sun's habitable zone. So while the recently confirmed Kepler findings are interesting and encouraging, I don't think we're quite ready to send the Welcome Wagon to our new neighbors.

And neither Jupiter's moon Europa nor Saturn's moon Enceladus would be considered to be in the Sun's habitable zone, although both are believed to have liquid water below surface ice. So this concept of a habitable zone seems very approximate, a good rule of thumb as we begin to catalogue and classify exoplanets, but certainly not a way to rule in or out which exoplanets might have life as we know it.

[DavidLeodis]

I first saw this APOD on March 4 2014 and found that the "Depicted above" link in the "Depicted above in artist's illustrations are twelve extrasolar planets" brought up a version of the image that had been updated on March 4 2014 and has 20 extrasolar planets. I thought (hoped) it would be of interest to report this update to the image used for the APOD.

[Ann]

I like the overall import of your comment Rob, and you express things the way science should operate in principle. However, when you consider the new branch of the sciences called Exobiology there doesn't seem to be a "conservative view of things."

I disagree. The publications of exobiology (the majority of which come from biologists) are as solid and robust as any other, and are typically cautious in advancing hypotheses that are hard to justify.

But when it comes to planets actually harbouring life, we still have an example of one, namely the Earth. What's worse, life arose on the Earth in the distant past, which is inaccessible to us. We can't go back there and find out the step-by step process when life actually got a foothold. We can't measure the average temperature or the temperature variations, the salinity of the water, the general chemistry, the atmospheric conditions, electrical discharges, or the amount of ultraviolet radiation that reached the surface of the water, or how deeply the ultraviolet radiation penetrated into the water. We can't measure what happened at the interface between the primeval liquid ocean and the atmosphere. We can't measure what happened at underwater hotspots of hydrothermal vents. We can't see if complex pre-biotic chemistry favored certain local spots and certain conditions there. We can't see if the first life forms quickly became extinct again. We can't see if basically the same life forms made a comeback after succumbing, or if the first life forms were replaced by others that were fundamentally different in some respects. We can't see if "life made several attempts" before it really triumphed, and we can't see if the Earth was truly lifeless between "intervals of life". We can't see if life ever truly disappeared from the Earth after it had appeared once. We can't know if the life forms that originally appeared were particularly hardy, and we can't know if "the life processes" in themselves were so hardy that they made new life forms appear even if the Earth had become sterile after the first life forms had become extinct. We don't even know if the first life forms or the basic "life processes" were comparatively weak, and if there was a sudden, unlikely mutation that suddenly made the life forms very robust.

There are so many things we don't know, and we can't learn about them by observing life on other planets, since we haven't found actual life on any other planets.

Compare with all the things that we can know about by observing the rest of the universe. We can learn about our own Sun by observing huge numbers of stars in our own galaxy. We can learn about our galaxy by studying huge numbers of other galaxies. We can learn about the expansion of the universe by observing that (almost) all other galaxies in the universe appear to recede from us. We can predict the collision between the Milky Way and Andromeda by observing huge numbers of other interacting galaxies in the universe.

But we have had other examples of one, too. Think about our solar system. For the longest time we didn't even know if other planets existed anywhere, and now we have found so many of them that we have reason to think that most (and just possibly all) stars have one of more planets. Could we have known that before we were able to observe extrasolar planets? I think we might have guessed it, for the simple reason that we already knew of so many different bodies and objects that were obviously in orbit around the Sun. Is it likely that so many bodies and objects could be in orbit around the Sun, if no other stars had any objects in orbit around them at all? I don't find that likely.

But when it comes to life, we haven't found it anywhere but on the Earth. I realize that it is possible to make some predictions about life on other planets solely based on what we know about life on the Earth. But since there are many things we don't know about how life came into existence and gained a firm foothold on the Earth, it is nevertheless hard to make very many predictions about life elsewhere.

Ann

[Chris Peterson]

But when it comes to planets actually harbouring life, we still have an example of one, namely the Earth.

But understand, when you review exobiology publications, only a tiny fraction even deal with other planets, or searching for life elsewhere. The vast majority are papers exploring lifeforms on Earth, exploring biogenesis, exploring alternate organic chemistries. Exobiology is, first and foremost, about understanding life along broader principles than just the examples we have.

[Ann]

In the order in which they are presented in today's APOD, the estimated mean temperatures in degrees C of these exoplanets (per Wikipedia) are:

Upper row: 23, -3, 70, -24, -34, 6
Lower row: 59, 0, 22, -65, -64, -92

I believe those are mean surface temperatures, not mean temperatures. Certainly, we have examples of lifeforms on Earth that can thrive at any of these, and we have a wider range of local temperatures on the Earth than this.

Are there really life forms on the Earth that thrive at a mean temperature of -92 C? And are there really many places on the Earth that have colder mean surface temperatures than -92 C?

Ann

[Chris Peterson]

Are there really life forms on the Earth that thrive at a mean temperature of -92 C? And are there really many places on the Earth that have colder mean surface temperatures than -92 C?

Colder mean temperatures? No. But the coldest temperatures are right around that, and have nearby life (such as below the surface). Europa is much colder on the surface, but is probably habitable not far beneath that. So it's no real stretch to presume that a planet with a - 92°C surface temperature could support Earth-like lifeforms.

[BMAONE23]

http://www.dailymail.co.uk/news/article ... years.html

Boil the 1mm creatures, freeze them, dry them, expose them to radiation and they're so resilient they'll still be alive 200 years later. Water bears can hack temperatures as low as -457 degrees, heat as high as 357 degrees, and 5,700 grays of radiation, when 10-20 grays would kill humans and most other animals. The animals can also live for a decade without water and even survive in space

The Tardigradewould have no problem at -92c

Click to view full size image

Click to play embedded YouTube video.

[Chris Peterson]

The Tardigradewould have no problem at -92c

A tardigrade at that temperature survives by becoming dormant. That isn't thriving.

[Beyond]

But it's a lot better than expiring!

[Chris Peterson]

But it's a lot better than expiring! :yes:

If it doesn't ever warm above freezing, however, "dormant" and "expired" are functionally the same

[chuckster]

There's been an "are we alone week" running on cable this week, that got me thinking, and I had a couple questions that maybe someone on here can help me with.
In one documentary, it was stated that, in our galaxy, red dwarfs outnumber main sequence stars by 3 to 1, so they represent numerous opportunities for things to not be so easy for life to arise. If red dwarves are so numerous, and ostensibly unlikely to host life, aren't double stars also highly unlikely to do so, and are even more numerous than the dwarves ? And how many of those red dwarves are orbiting a barycenter with a main sequence star ? I remember something about Poincarre's three-body problem being unsolvable and a prime example of nonlinear dynamics, etc, so does that mean double (or more) star systems are, by definition, not eligible to harbor planets in a Goldilocks zone, because they would never allow orbiting planets to settle into stable orbits at all ? To me, it seems that single-star systems are the only happy hunting ground, but everything I've seen and read so far is silent on this.
Finally, in all the talk of exoplanets we don't hear much about good ol' Alpha Centauri. Granted, it's a two- and possibly three- star system, but at 4.5 light years, can't we get a better look at what's going on out there somehow ? It's our closest neighboring star system, so what say the exoplanet hunters ?

THANK YOU

[geckzilla]

There's been an "are we alone week" running on cable this week, that got me thinking, and I had a couple questions that maybe someone on here can help me with.
In one documentary, it was stated that red dwarfs outnumber main sequence stars by 3 to 1, and therefore life has had a lot of chances to arise on any orbiting planets during the dying stages of many stars. OK, but what is the ratio of double star systems to single systems, like ours ? If red dwarves are so numerous, and ostensibly unlikely to host life, aren't double stars also highly unlikely to do so, and are even more numerous than the dwarves ? And how many of those red dwarves are orbiting a barycenter with a main sequence star ? I remember something about Poincarre's three-body problem being unsolvable and a prime example of nonlinear dynamics, etc, so does that mean double (or more) star systems are, by definition, not eligible to harbor planets in a Goldilocks zone, because they would never allow orbiting planets to settle into stable orbits at all ? To me, it seems that single-star systems are the only happy hunting ground, using any method of detection.
Finally, in all the talk of exoplanets we don't hear much about good ol' Alpha Centauri. Granted, it's a two- and possibly three- star system, but at 4.5 light years, can't we get a better look at what's going on out there somehow ?

THANK YOU

The Universe's occupants comes in many shapes and sizes so you'll just have to use your imagination on all of the many configurations out there. A double star system doesn't have to preclude life if the stars are positioned correctly. Some interesting dynamics are possible with such a system. I imagine a larger star with a very small star, sort of like our system except replace Jupiter with a small star. If everything is just right, maybe you could have more than one habitable zone in such a system. Maybe that makes it twice as likely to harbor life.

Alpha Centauri... we can't even see Pluto and its angular diameter is 0.063″ – 0.115″ depending on its current orbital position. Alpha Centauri A is 0.007″, which is only 6% as big as Pluto under Pluto's best possible viewing conditions. Space between objects is unfathomably big. I know it's getting hackneyed to say things like that but go ahead and click that link.

[chuckster]

I guess I got caught up in the "blocking in" of the question of exoplanets in greenzones. It's one thing to assay the relative populations of star types in the Milky Way, and another for the universe at large. I read someplace that, for Jupiter to have gravitationally collapsed and begun fusing, it would have had to be 11 times larger.

I've also read that when we look at Alpha Centauri from Earth, we see a single point of light that is really both of the Sunlike stars buried in each other's glare. But as far as I know, for exoplanet search purposes, that doesn't matter. All the currently listed exoplanets orbit stars a LOT further away then Alpha Cen. If the transit method only works when our ecliptic is coplanar with the target star's, then what about the wobble method ? Isn't that a Doppler shift measurement ? Has this been done on the Alpha Cen system ?

[geckzilla]

There are a couple of references in the Alpha Centauri's Wikipedia article regarding the existence of a possible planet. See the fourth paragraph from the top. For the wobble method to work, the wobble has to be significant enough to be detectable. Maybe something is there but it's not detectable. They looked at it, said there was one there, looked again, then said maybe it's not there. For Alpha Cen, any planets may be lost in the noise for a while yet.

[chuckster]

Thanks for the insights ! I flew the radius of the solar system in that link you provided - I think it brings home the meaning of distance in a more "tangible" (i.e. finger on the right arrow key) way than most. The comments along the way were entertaining and mind-expanding. Now I'll go read that link on Alpha Cen.

Let me leave you with another insight, from a fictional physicist in the novel "Fiasco" by Stanislaw Lem :

"Physics, my friend, is a narrow path drawn across a gulf that the human imagination cannot grasp. It is a set of answers to certain questions that we put to the world, and the world supplies answers on the condition that we will not then ask it other questions, questions shouted out by common sense. And common sense? It is that which is understood by an intelligence using senses no different than those of a baboon."

[Ann]

chuckster wrote (before editing):
In one documentary, it was stated that red dwarfs outnumber main sequence stars by 3 to 1, and therefore life has had a lot of chances to arise on any orbiting planets during the dying stages of many stars.

This is a good and interesting point. On one hand, the surfaces of planets orbiting in the habitable zones of red dwarfs are problematic places for life (not impossible, but problematic). On the other hand, life has a long, long, long time to make it either on the surface or below the surface of such a planet.

Since the habitable zone of a red dwarf is so small, a planet orbiting there will become tidally locked, so that one side of the planet always faces its sun. The opposite side of the planet will always face away from the star. This arrangement should be disadvantageous for life on the surface of such a planet, although it may not at all be impossible. Of course, life below the surface may not be affected at all.

And the long, long, long-lasting existence of a red dwarf will certainly give life many chances to find a way to survive on the surface or below the surface of a planet in orbit of such a star.

But suppose it really does take a long time for life to find a way to make it on the surface of a planet orbiting a red dwarf. If so, life may not have had enough time to establish itself on the surface of a planet of a red dwarf. Remember that the universe is "only" about 14 billion years old.

Ann