APOD: A Dangerous Sunrise on Gliese 876d (2012 Apr 29)

[APOD Robot]

A Dangerous Sunrise on Gliese 876d

Explanation: On planet Gliese 876d, sunrises might be dangerous. Although nobody really knows what conditions are like on this close-in planet orbiting variable red dwarf star Gliese 876, the above artistic illustration gives one impression. With an orbit well inside Mercury and a mass several times that of Earth, Gliese 876d might rotate so slowly that dramatic differences exist between night and day. Gliese 876d is imagined above showing significant volcanism, possibly caused by gravitational tides flexing and internally heating the planet, and possibly more volatile during the day. The rising red dwarf star shows expected stellar magnetic activity which includes dramatic and violent prominences. In the sky above, a hypothetical moon has its thin atmosphere blown away by the red dwarf's stellar wind. Gliese 876d excites the imagination partly because it is one of the few extrasolar planets known to be in or near to the habitable zone of its parent star.

<< Previous APOD

Discuss Any APOD

Next APOD >>

[/b]

[neufer]

A Dangerous Sunrise on Gliese 876d

Gliese 876d excites the imagination partly because it is one of the few extrasolar planets
known to be in or near to the habitable zone of its parent star.

• How nice.

[Ann]

Interesting. It raises the question whether or not red dwarfs are good hosts for life-bearing planets.

The first book I read that touched on the subject was Isaac Asimov's Extraterrestrial Civilizations from 1979. In that book, Asimov argues that it is no coincidence that we are in orbit around a G-type star. G-type stars have the right mass to produce a habitable zone that is far enough from the star not to induce tidal locking in the planets that orbit them. They also have a sufficiently long main sequence life time to give biological life many billion years to evolve on a planet inside their habitable zones.

Asimov said that red dwarfs would not do as hosts for life, simply because the planets would have to orbit so close to the star that they would become tidally locked. Also, said Asimov, it's never a good idea to be very close to a sun of any kind, because any star might have dangerous outbursts.

Lately, however, my impression is that red dwarfs have become extremely popular among planet hunters who hope to find life away from the Earth. The reason for the popularity of the red dwarfs is that they are ubiquitous and extremely long-lived. If they will do as hosts for life, the odds of finding life in space increases dramatically. Astronomers have therefore run several computer simulations to disprove the life-unfriendliness of red dwarfs. They have, for example, run computer simulations to disprove the life-unfriendliness of tidally locked planets inside the habitable zone of red dwarfs, and they have done a fairly good job of disproving the necessarily life-killing nature of tidal locking. To me, however, trying to prove the life-friendliness of red dwarfs by disproving the life-unfriendliness of them feels a bit like cheating. Of course that's just my impression.

I have another argument against the red dwarfs as hosts for life. In his book Planet Quest Ken Croswell writes that 80% of all stars in the Milky Way are M-type red dwarfs. 9% are K dwarfs. 5% are white dwarfs. 4% are G-type main sequence stars, and 2% are "others" (F-, A-, B- and O-type main sequence stars, red giants, other types of giants and supergiants and others).

Okay. Imagine that you have a shallow tub of some kind, and you fill it with bottles of soft drinks. You put a hundred bottles of soft drinks in the tub. Eighty of the bottles contain Coke. Nine are Fanta. Five are Sprite. Four are Root Beer. Two are a kind that you haven't heard of before.

Imagine that you are blindfolded and asked to put your hand in the tub and grab a bottle. What is the chance that you will grab a bottle of Coke? It's 80%. What is the chance that you will grab a bottle of Root Beer? It's 4%. You are twenty times more likely to have a Coke than a Root Beer.

That's my point. If M-type red dwarfs are equally good hosts for life as G-type stars like the Sun, then we have indeed beaten the odds by being in orbit around a G-type stars like the Sun instead of an M-type red dwarf like Gliese 876. I think it is just too improbable that an unusual star like the Sun would be our star, instead of a "dime a dozen" star like Gliese 876, if both are equally good hosts for life.

So my point is, I don't believe that M-type red dwarfs are very good hosts for life. But I guess and hope that time, and continuous efforts by astronomers, will tell.

Ann

[Boomer12k]

Chronicles of Riddick....


:---[===] *

[Ann]

Of course, a problem here is how we define "life". To most people, "life" probably means people and higher animals. To astronomers and astrobiologists, it might mean simple bacteria-like life forms.

M-type red dwarfs may be good hosts for simple bacteria-like life forms, for all I know. How good would they be for life forms like ourselves, though?

Let's consider the Drake Equation:

N = R* x fp x ne x fℓ x fi x fc x L

R* = the average rate of star formation per year in our galaxy
fp = the fraction of those stars that have planets
ne = the average number of planets that can potentially support life per star that has planets
fℓ = the fraction of the above that actually go on to develop life at some point
fi = the fraction of the above that actually go on to develop intelligent life
fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L = the length of time for which such civilizations release detectable signals into space

Let's have fun with the Drake Equation and try to figure out the number of M-type red dwarfs that may host technological civilizations, if we are extremely optimistic. I remember reading about someone who was so very extremely optimistic, so let's copy that person.

R* is the average rate of star formation per year in our galaxy. But when it comes to the red dwarf stars, every M-type dwarf that was ever born in our galaxy still exists, unless a few of them have been eaten by black holes or been flung out of our galaxy or something. None of them have died a "natural death". Because of that I'm not going to consider star formation here, but the number of M-type red dwarfs in our galaxy. How many are there?

Let's suppose that there are 200 billion stars in our galaxy. If 80% of them are M-type red dwarfs, then there are about 160 billion red dwarf stars in our galaxy.

Now let's consider the number of them that have planets. Lately planet hunters have been very optimistic about planets around stars. My impression is that they believe that at least half of all stars have planets. Let's say that 50% of all M-type red dwarfs have planets. That means that 80 billion red dwarfs have planets.

How many of the red dwarfs stars have planets in their habitable zones? I believe planet hunters are pretty optimistic about that, too. Let's say that at least 10% of those that have planets have planets in their habitable zones. That would mean 8 billion red dwarf stars with planets in their habitable zones.

How many of the planets in habitable zones will develop life at some point? The optimistic guy I remember reading about claimed that most planets inside a planet's habitable zone will develop life at some point. Admittedly, if we are talking about very simple bacteria-like life forms, many planets inside habitable zones could be making them. Let's say 50% of the planets inside a stars habitable zone will develop life. That means there could be 4 billion red dwarfs with planets that have, or have had, life on them.

How many of the planets with simple bacteria-like life forms will see the evolution of complex life forms? The optimistic guy I remember said that either 50% or 10% of the planets with bacteria would see the rise of complex life, too. I'm skeptical. But let's go with the 10%. That would make 400 million planets with complex life forms in orbit around red dwarfs that have evolved complex life forms.

Hmmmm. I'm really feeling skeptical here.

What about intelligent life? The optimistic guy I've referred to talked about life as if it will naturally evolve into ever-higher intelligence. He claimed that either 50% or 10% of all planets with complex life forms will evolve species with an intelligence similar to ours. I don't believe it, but for the fun of it, let's go with the 10%. That would make 40 million red dwarfs with planets populated by beings whose intelligence is comparable to ours. I don't believe it.

How many of the planets with intelligent species would see their intelligent species create technological civilizations? Mr Optimist, they guy I've been talking about, said that either 50% or 10% of the intelligent species would build technological civilizations. For the fun of it, let's go with the 10%. That would make 4 million technological civilizations in orbit around red dwarfs.

Of course, we haven't considered how long a technological civilization lasts. If they have short life times there will most certainly not be 4 million red dwarfs orbited by planets with technological civilizations today (whatever we mean by "today", since "now" can really only exist "here").

But I'm just saying - what are the chances of complex, intelligent and eventually technological beings like ourselves coming to existence on a planet like Gliese 876d? Surely that hasn't happened 4 million times in our galaxy.

Ann

[nstahl]

Quick reply: Why Not?

And I'm curious about the philosophy/religion behind your "let's reach into a tub and find a planet for humans" model of why we're here. "Here" being on earth around the sun, not "in existence".

[owlice]

Please use PM/another means for any discussion of religion; thanks.

[rstevenson]

... If M-type red dwarfs are equally good hosts for life as G-type stars like the Sun, then we have indeed beaten the odds by being in orbit around a G-type stars like the Sun instead of an M-type red dwarf like Gliese 876. I think it is just too improbable that an unusual star like the Sun would be our star, instead of a "dime a dozen" star like Gliese 876, if both are equally good hosts for life. ...

I'm not sure your logic is correct here. We haven't "beaten the odds" by being here. On the contrary, the odds were stacked in our favour or we wouldn't be here to talk about it. Think of it this way... if you buy a ticket in a lottery where the odds against winning are 13 million to one, and you win, you haven't beaten the odds -- though most people would put it that way. In fact your ticket was the winning ticket when you bought it. The odds of that ticket winning were 100%. (Now if only I could figure out which ticket had those odds and buy it... .)

Or imagine this... there is another technical civilization elsewhere in the Milky Way, and in that civilization there is a forum in which people are discussing the likelihood of life evolving around stars other than red dwarf stars like their own. If they don't know about us or any other such civilization orbiting a G-type star, then they may asign a very low probability to our existence simply because G-type stars are so "unusual." Nevertheless, we exist.

Your conclusion above is weighted with words such as "improbable" and "unusual", but our Sun is not unusual in being a G type star, it is a G-type star. It's just that there are relatively few of them in the Milky Way, and that says nothing about how probable or improbable it is that it is our star.

There's simply no way to argue the case for the probability of life arising on other planets around other stars given our current state of knowledge. We only have a data set of one, and in that data set it is 100% probable that life would arise around this G-type star on this rocky planet. To draw any more of a conclusion than that, we'll have to wait for more data sets. (But the Drake equation is fun to play with.)

Rob

[Ann]

I can't put it better than I did in my example with the tub filled with a hundred bottles of soft drinks. Eighty of the bottles are Coke and four are Root Beer. I think that if you reach into the tub when blindfolded and happen to pick one of the bottles of Root Beer, then you really have beaten the odds. You would be so much more likely to pick a bottle of Coke.

If M-type red dwarf stars are, on average, equally good hosts for life-bearing planets as G-type main sequence stars like the Sun, then it is pretty remarkable that our Sun is a G-type main sequence star and not an M-type red dwarf. Or, to put it differently, then it is pretty remarkable that we "happened to pick" a G-type star to orbit, when there are so many more "equally suitable" M-type red dwarfs out there.

On the other hand, if M-type red dwarf stars are generally poorer hosts for life-bearing planets than G-type main sequence stars, then it may be no coincidence at all that our Sun is the way it is. We could be here, in orbit around the Sun, because we - and beings broadly similar to us - could not possibly survive on a planet like Gliese 876d.

I need to stress, once again, that we should define what kind of life we are talking about when we discuss "life". Can we accept the idea that simple bacteria-like life forms might form quickly and easily, but that only a miniscule fraction of them will ever evolve into higher life forms? Or do we postulate the idea that at least 10% of planets with bacteria-like life forms will see the ascent of complex life? It could be that M-type red dwarf stars are equally good hosts for bacterial life as G-type main sequence stars. But what about life forms broadly similar to ourselves? Are we here, in orbit around the Sun, because the Sun is massive enough to have its habitable zone sufficiently far away from itself that the Earth has not become tidally locked in its orbit around the Sun, so that we can enjoy clement conditions here on Earth? Could beings similar to ourselves evolve on a planet like Gliese 876d?

I find the thought improbable. But like you said, Rob, of course I can't know.

Ann

[luigi]

Nice APOD, to think about on a Sunday morning

I think that if life were possible in red dwarves systems then life would be so common in the Milky Way that it would had been be very hard to avoid being visited, contacted or radioed by another civilization.
The fact that we are still alone makes me think that either life can't evolve around a red dwarf at all or if it can then intelligent life is far more uncommon that what we think.

The illustration is fantastic.

[ritwik]

there could be life forms inside a star itself !!few people know the fact that micro organisms thrive inside molten lava !:!: there could be garguantuan living things flying through interstellar space nesting between star clusters even if there's nothing like that it would be more astounding

[Chris Peterson]

That's my point. If M-type red dwarfs are equally good hosts for life as G-type stars like the Sun, then we have indeed beaten the odds by being in orbit around a G-type stars like the Sun instead of an M-type red dwarf like Gliese 876. I think it is just too improbable that an unusual star like the Sun would be our star, instead of a "dime a dozen" star like Gliese 876, if both are equally good hosts for life.

This is just a restatement of the anthropic principle, and represents a logical fallacy in this context.

[eltodesukane]

there could be life forms inside a star itself !!few people know the fact that micro organisms thrive inside molten lava !:!: there could be garguantuan living things flying through interstellar space nesting between star clusters even if there's nothing like that it would be more astounding

"micro organisms thrive inside molten lava" No.

[Eduk8tr]

Hi All,

Just curious as to why this image is the same that was posted back on May 21, 2008. Thanks.

[eltodesukane]

Even though earth is 4500 millions years old, and earth life is almost as old, there have been humans for only (at most) 10 millions years.
This shows that evolution of advanced intelligent life from primitive microorganisms is not a quick straightforward process.

[Chris Peterson]

I think that if life were possible in red dwarves systems then life would be so common in the Milky Way that it would had been be very hard to avoid being visited, contacted or radioed by another civilization.

"Life" implies neither civilization nor technology. There has been life on Earth for around 80% of its existence. Life formed here, and continues to exist here, under conditions that are readily found outside so-called "habitable zones", and under conditions we would generally consider pretty hostile to life as we know it. There is some experimental evidence (far from conclusive, of course) that life develops easily. We know that the chemicals are ubiquitous in the Universe. So it's perfectly reasonable to think that the majority of planets far enough from their stars to have some cool areas also have life on them.

All we know about life is that it evolves. We know very little about how high intelligence evolves, because it has only happened once, in response to uncertain evolutionary forces, and with an unknown element of chance involved. There's a good argument to be made that technological intelligence is a dead end, and is inevitably short lived. But even if it's not, and most life ultimately evolves in that direction, we'd still expect (given the time statistics on Earth) only a tiny fraction of planets would host technological civilizations. And no matter how advanced, they're still subject to the laws of physics, which means they're largely confined to their own planetary system, or at most a tiny zone of stars around it. So there's really no reason we would expect to be aware of them.

[bystander]

Just curious as to why this image is the same that was posted back on May 21, 2008. Thanks.

See the APOD FAQ.

[Guest]

[quote="Ann"]I can't put it better than I did in my example with the tub filled with a hundred bottles of soft drinks. Eighty of the bottles are Coke and four are Root Beer. I think that if you reach into the tub when blindfolded and happen to pick one of the bottles of Root Beer, then you really have beaten the odds. You would be so much more likely to pick a bottle of Coke.

Incredibly interesting discussion and speculation however, isn't this a case of the act of observing changing the equation? Since the bottles have different shapes, the person grabbing the drinks could cheat. The grabber could also change their mind and switch bottles before retracting it.
That changes the odds. Once the grabber makes a selection, whatever bottle they hold is now at 100 percent chance of being selected. It is not so much beating the odds as playing with odds.
Also if you figured the odds of the Universe(s) spontaneously existing is incredibly low until you realize that since it does exist the odds are 100 percent that the Universe(s) would spontaneously come about.

[Eduk8tr]

Just curious as to why this image is the same that was posted back on May 21, 2008. Thanks.

See the APOD FAQ.

Thanks.

[nstahl]

... If M-type red dwarfs are equally good hosts for life as G-type stars like the Sun, then we have indeed beaten the odds by being in orbit around a G-type stars like the Sun instead of an M-type red dwarf like Gliese 876. I think it is just too improbable that an unusual star like the Sun would be our star, instead of a "dime a dozen" star like Gliese 876, if both are equally good hosts for life. ...

That's not at all how I see it. Life clearly developed here, and if you rule out panspermia and seeding by aliens, life arose here. Since it happened here I have no problem imagining it's happened in a lot of similar situations. We don't know enough about conditions in which life can arise and exist to say whether it can happen on red dwarf planets or not, but if it can then surely there are scads of red dwarf planets with life. And in no way does life existing here rule out life existing around a red dwarf; in fact I see no way in which life existing here could impact the probability of life existing around a red dwarf. They are independent events, at least for practical purposes.

[gdreiber]

1. Evolution is random mutations on random organisms in random environments.
2. Human scientist equate brain to body mass ratio and an animals train-ability to intelligence; thus dolphins and apes get high marks for intelligence. The chance that an organism will randomly evolve to into roughly a Humanoid intelligence on another planet: Zero.
3. The ability to recognize, understand and communicate with an alien intelligence, maybe the plus side of zero but not much.

[Mactavish]

I can't put it better than I did in my example with the tub filled with a hundred bottles of soft drinks. Eighty of the bottles are Coke and four are Root Beer. I think that if you reach into the tub when blindfolded and happen to pick one of the bottles of Root Beer, then you really have beaten the odds. You would be so much more likely to pick a bottle of Coke.

Ann

Nonsense. The reason you would pick a Coke is simply because 'Things go better with Coke!'

[Chris Peterson]

1. Evolution is random mutations on random organisms in random environments.
2. Human scientist equate brain to body mass ratio and an animals train-ability to intelligence; thus dolphins and apes get high marks for intelligence. The chance that an organism will randomly evolve to into roughly a Humanoid intelligence on another planet: Zero.
3. The ability to recognize, understand and communicate with an alien intelligence, maybe the plus side of zero but not much.

You could as easily say that human-like intelligence is inevitable. Certainly, one path that is consistently seen in evolution is a trend towards larger, more effective brains. That's a survival trait, especially amongst predators.

The bottom line is that while we're on somewhat firm ground suggesting that life is common, we really don't have enough information to make any predictions at all about highly intelligent life.

[rstevenson]

... The bottom line is that while we're on somewhat firm ground suggesting that life is common, we really don't have enough information to make any predictions at all about highly intelligent life.

For the likelihood of intelligence we may have at least one other data point to think about. Right here on planet Earth a form of life existed on land in a wide array of sizes, shapes and abilities for a period of about 135 million years. Yet, so far as we know, they never developed intelligence beyond that needed for survival. (Or at least they left no evidence of their intelligence which could survive the intervening 65 million years. I wonder if anything we've done would survive that long.) But humanity developed from small mammals to Moon walkers in half that time. I can draw no conclusions from that, but it's interesting to think about.

Rob

[Ann]

... If M-type red dwarfs are equally good hosts for life as G-type stars like the Sun, then we have indeed beaten the odds by being in orbit around a G-type stars like the Sun instead of an M-type red dwarf like Gliese 876. I think it is just too improbable that an unusual star like the Sun would be our star, instead of a "dime a dozen" star like Gliese 876, if both are equally good hosts for life. ...

That's not at all how I see it. Life clearly developed here, and if you rule out panspermia and seeding by aliens, life arose here. Since it happened here I have no problem imagining it's happened in a lot of similar situations. We don't know enough about conditions in which life can arise and exist to say whether it can happen on red dwarf planets or not, but if it can then surely there are scads of red dwarf planets with life. And in no way does life existing here rule out life existing around a red dwarf; in fact I see no way in which life existing here could impact the probability of life existing around a red dwarf. They are independent events, at least for practical purposes.

The point I was trying to make is that an M-type star has its habitable zone very close to itself. Remember that the the heat and light that a star emits is not linearly proportional to its mass. Proxima Centauri, the very nearest star apart from the Sun, has a mass about one tenth that of the Sun, but its visual light output is about one part in eighteen thousand that of the Sun. It does a bit better when it comes to infrared light, but even there it only produces one part in 600 that of the Sun.

Conclusion? In order to receive enough heat from Proxima Centauri, a planet would have to cosy up very close to that star. But if the planet was so close to Proxima Centauri, the planet would feel the gravity of its red dwarf sun much, much more than we feel the gravity of the Sun. Remember that while Proxima only produces one part in 600 the heat of the Sun, it holds one tenth the mass of the Sun.

The gravity of Proxima Centauri would cause the planets inside its habitable zone to become tidally locked, or at the very, very least to rotate extremely slowly. That might lead to very strange weather on that planet.

I also think that Proxima Centauri has had a couple of bright ultraviolet flares from its "surface". A planet that orbited very close to Proxima Centauri would be strongly affected by those flares.

My point is that any star is dangerous, so that you don't want to orbit too close to it. But a red dwarf star has such a small habitable zone so close to itself that a planet inside the red dwarf's habitable zone will have to orbit very close to that star.

Like Chris said, we don't know how life elsewhere might adapt to its conditions. It could well be that alien life forms might thrive in living conditions that seem absolutely impossible to us.

Nevertheless, I remain unconvinced that life just adapts to whatever conditions it meets. Perhaps bacterial life can adapt to an incredibly wide range of circumstances, but I doubt that complex life can repeat that trick. Consider Mars. Very many scientists seem to believe that there might be bacterial life on Mars. That is because we can see that there is water on Mars, and we have good reasons to believe that underground water on Mars might very well offer acceptable living conditions for bacteria.

But what about more complex life forms? I have never heard a single astrobiologist suggest that there might be little green men on Mars after all, but in caves underground. No one seems to believe that large complex life forms - as large as earthworms, say - can survive on Mars, or below the Martian surface. What reason do we have to think that large complex life forms can thrive on a planet that has to cosy up very close to its red dwarf star and be subject to that star's strong gravity (strong when you are close to it) and its various outbursts?

Ann