The Questionable Authority

Earlier today, a team of researchers lead by noted exoplanet hunter Michel Mayor announced a pair of blockbuster discoveries – the lowest mass planet yet discovered orbiting another star, and a new analysis suggesting that another, previously discovered planet is orbiting that same star within the theoretical “habitable zone” where liquid water can exist. Both of these discoveries are huge, but for different reasons.

The small planet, now known as Gliese 581 e, may weigh in at only about 1.9 Earth masses. That makes it the smallest exoplanet yet discovered. Gliese 581 is a red dwarf star, with a mass about a third of the suns. Gliese 581 e is orbiting the star at about 0.03 AU (1 AU = distance from here to the sun), and completes an orbit in just over three days.

As cool as it would be to live someplace where I’d already be far older than Methuselah, Gliese 581 e probably isn’t a very hospitable place to live. It’s way too close to the star – it’s probably an oversize, overcooked Mercury. Even if it’s not a vacation spot, it’s still a really cool find – it shows that scientists are getting much better at finding smaller exoplanets.

Cool as that is, it pales compared to Mayor’s team’s second announcement. They were able to refine the estimates for the orbit of Gliese 581 d – one of three other planets that had already been discovered orbiting Gliese 581. It appears that this planet is well within the star’s habitable zone, which means that liquid water – and possibly life – can exist there.

The question is, does this mean that we should dust off the welcome mat?

Probably not. Gliese 581 is only about 20 light years away, and if there was an advanced civilization there right now, I suspect that the SETI folks would probably have seen some indications by now (if they use radio, anyway). But maybe they don’t, for whatever reason. Could there be life there, and might they reach space?

Obviously, we can’t do any more than speculate about what kind of life might evolve on a planet like Gliese 581 d. Our speculations are, at best, going to be almost completely uninformed, but that’s never stopped people from speculating before, so I certainly see no reason to let it stop us now.

Let’s start by looking at what we know about the planet. That’s something that’s really not going to take us very long. We know, according to the press release, that it’s at least 7.7 Earth masses, and it orbits the star about every 66 days.

I told you that wouldn’t take long. Now, let’s speculate.

One of the many physical factors that’s important to life is gravity – particularly when we’re going to think about the potential for flight. We know that Gliese 581 d is a lot more massive than Earth, but we don’t actually know what the planet’s surface gravity is. In order to know what the surface gravity of the planet is, we need to know its radius. At the moment, we’ve got no idea what Gliese 581 d’s radius is, because we also don’t know what its density is.

Not all planets are equally dense. The planet we’re on is about 5.5 times as dense as water. Venus and Mercury are a little less dense than we are, and Mars is a lot less dense – it comes in at about 3.9. If 581 d is a rocky planet, it’s density is probably somewhere in that range. The research team that discovered the planet doesn’t think that it’s a rocky one, though. They say that, given how massive it is, it’s more likely to be an icy-type planet. In our solar system, Pluto and Saturn’s moon Titan fall into this category of object. Their densities are about 2.

That gives us quite a range of densities to look at, but we can use the possible densities to work out the possible size of the planet. If we make some simplifying assumptions (like pretending that the planet is perfectly spherical), it’s fairly simple to figure out. We know the mass of the planet, and we know that volume equals mass divided by density, so we can punch in a range of densities and get a range of volumes.

If we dust off our 10th-grade geometry, we can figure out what the radii for spheres with each of those volumes. Once we’ve done that, we can figure out the surface gravity that goes with each of the possibilities. That’s also fairly simple (g = m/r^2, where mass and radius are expressed relative to Earth’s). I went ahead and looked a a range of densities that ran from 0.55 (0.1 Earth-density) to 11 (2 Earth-density).

i-877b9effd5de334aa6c01a1e4c1eeebe-581d1-tm.jpg

As you can see, that’s quite a range. Here’s the bottom line, though: the Earth, as I’ve mentioned, is the densest of the rocky planets in our solar system. If 581 d is as dense as the Earth, it’s got just about twice our surface gravity. However, if the scientists who discovered the planet are right, and it has a density similar to that of Pluto or Titan, (~0.3 of Earth’s density), it probably has a surface gravity very similar to ours.

In that case, though, there’s a decent chance that it’s a water world – a planet that’s one giant ocean. Not only do these planets have no continents, islands, or even rocks breaking the surface, they don’t even really have an ocean floor as we understand the concept. The water simply gets deeper and deeper until the pressure increases to the point where the water becomes ice. There’s no solid evidence to suggest that Gliese 581 d is an ice planet, but its size is right in the predicted range for an ocean world. Given that the estimated age of the Gliese 581 system is in the neighborhood of seven billion years, it’s certainly plausible that 581 d started out as an outer solar system ice world, and has since migrated into the habitable zone.

It’s fun to speculate about what sort of intelligent life might evolve on a water world. What would their myths and legends involve, and how would their environment shape their view of the universe? Would they have the same desire to reach for the stars that we do?

Maybe they do, but haven’t made it yet. Or maybe they did, but they’re long gone.

If you’re at all familiar with anything that has to do with the search for extraterrestrial intelligent life, you’re probably familiar with the Drake Equation, which is sometimes used to estimate the number of intelligent species out there in the galaxy (and is more appropriately used to remind us of just how much more we need to learn to be able to make anything like a reasonable guess about that number). One of the unknowns in the Drake Equation is L: the length of time that an advanced civilization lasts, on average. Even if the value for L is high – on the order of 1,000,000 years – it’s a small window when we’re talking about planets that have been around for billions.

Gliese 581 isn’t very far from us – at least in galactic terms. It’s only about 20 light years away. We’re clearly not going to go there in the immediate future, but it would not surprise me at all if we do someday, some time in the next few generations.

I wonder if we can even imagine what might be there when we arrive.

Hat tips: Steinn and Ethan.

Comments

  1. #1 dzdt
    April 21, 2009

    it would not surprise me at all if we do someday, some time in the next few generations.

    Do you mean, launch a probe in that direction sometimes in the next few generations? The problem is, it is going to take a lot longer than 20 years to get there. The fastest spacecraft we’ve launched so far [Voyager II, after picking up speed from the giant planet encounters] reached a speed of about 0.005% of the speed of light. At that speed it would take 375000 years to get to Gliese 581. Even if you figure we can get a factor of 1000 more velocity than that (a factor of 1 million energy required for the same mass probe!!!), that is still 375 years of travel time. For context, 375 years ago Isaac Newton had not yet been born.

  2. #2 Glendon Mellow
    April 21, 2009

    There’s a lot of bloggy ink about this announcement today, and I really enjoyed your post in particular.

    A pressurized water-ice boundary sounds exciting. I keep imagining chemical pockets and bubbles.

  3. #3 Hercolubus
    April 22, 2009

    Planet Gliese 581c and Planet X/Eris/Nibiru – Nibiru is one of many Planets that orbit a dark star or (Brown Dwarf). This Dark Star has five minor planets, the sixth an Earth-sized Homeworld, and the seventh the planet or object we call Nibiru… it is the physical link or “ferry” between our solar system and the dark star system of it’s Brown Dwarf Star:
    http://cristiannegureanu.blogspot.com/2009/01/astronomers-discover-earth-like-planet.html

  4. #4 johnstreeter
    April 22, 2009

    If it is a water planet, then this gets into a interesting thought process about the possible liquid depth before the ice forms; based on the visible light albedo/ir albedo of a water planet and the resultant water temperature, as well as the odd infrared dwarf light frequencies…would it be a steam planet? and maybe weird sun tidal effects? i wonder how spherical such a planet would really be?
    it boggles the mind!

  5. #5 larry
    April 22, 2009

    Earth to Gliese 581 e
    “SOS. We are in a recession. SOS. We are in a recession. SOS.”

    Gliese 581 e to Earth
    “Some scientists’ jobs are saved, they saw us, they won’t be laid off. Over.”

  6. #6 Joseph Dunphy
    April 22, 2009

    Wondering if you’re being unduly optimistic. How nutrient rich are the waters of such a planet likely to be? Wouldn’t the ice, forming under pressure, come between any bottom muck and the water, keeping upwellings from bringing nutrients up from the deep, and wouldn’t the deep would be an awfully long way down?

    Life can’t evolve out of just water. With a deficit of energy from a red dwarf sun that, however generous with the heat, wouldn’t be as generous with the light – light that I recall is expected to be passing through a deeper atmosphere’s than ours (581 d being placed into the life belt and 581 c removed because of the anticipated greenhouse effects of their respective atmospheres) and thus more greatly dimmed by the passage – we would be looking a dimly lit world. It would be one in which the light would be greatly separated from any nutrients, raising the question of how good a chance life would have of arising at all, wouldn’t it?

    But let’s say it does. Let’s even have it evolve into something intelligent. How is it even going to enter anything akin the stone age, much less move beyond it? Even on earth, which has far more light energy per unit of surface area to keep an ecosystem running – and let’s not forget that even tubeworms need oxygen from the surface – no ice layer at the bottom to serve as a barrier to the entrance of nutrients from the bottom into the water – the bulk of the life is at the surface, with the deep sea dwellers usually living on the crumbs that drift from above. Right?

    Where are the Gliesans going to find materials to work, when even the nearest rock is – what – maybe 50 miles down, under pressure equivalent to that which would be found at the bottom of a 100 mile deep ocean on earth? So, admitting that this is not my field, and that I could easily have missed something, I’m wondering if maybe the existence of any kind of civilization on that planet would be nearly inconceivable.

  7. #7 Jérôme ^
    April 22, 2009

    « The water simply gets deeper and deeper until the pressure increases to the point where the water becomes ice. »

    I always thought that the phase diagram for water had a reversed slope for the solid/liquid border (which means that you can’t freeze water by pushing on it; on the contrary, this melts ice). Perhaps this becomes false at huge pressures.

  8. #8 Mike Dunford
    April 22, 2009

    @Jérôme:
    There’s a pdf of a paper that discusses the predicted properties of an ocean planet available at arXiv. They’ve got a detailed phase diagram as their Figure 2.

    The slope of the solid-liquid border is negative until about 0.2 GigaPascals (~2,000 atmospheres). At that point, it becomes sharply positive. The authors of the ocean planet paper predict that a surface temperature of about 7C will lead to an ocean that’s ~78km deep. At the bottom, where the water turns to ice, the predicted water temperature is ~35C and the pressure is 1.1 GPa. (~10,000 atm).

    @Joseph:
    A number of cliches (and one spectacular Princess Bride quote) come to mind with the word “inconceivable”.

    I spent quite a while thinking about this one yesterday. I can think of a few different plausible scenarios (for a given value of “plausible” anyway) that might lead to the development of some sort of civilization (perhaps based first around plant products, then organic chemistry). I can even conceive of a dry land type culture there – maybe the bulk of the surface of the ocean has become covered with a giant stromatolite-like mat, and organisms evolved to live on that surface. Realistic? Who knows. Pure, unadulterated speculation? Absolutely. But when it comes to life, I don’t think anything is nearly inconceivable.

  9. #9 Tim
    April 22, 2009

    You guys are looking in the wrong place.

    http://www.nowpublic.com/strange/cnn-coverage-2009-national-press-club-x-conference-about-ufo-s-edgar-mitchell

    Be intellectully honest and look into this rather than shoot the same replies back I’ve been reading for 30 years. Cheers.

  10. #10 anon
    April 22, 2009

    Our best option for the near term (other than continued observation) would be a “Starwisp” robot probe, as proposed by Robert L. Forward — extremely light-weight “sail” propelled by a big laser, or battery of big lasers.

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

    The baseline design for this, “travelling at 11% of the speed of light … requires 40 years before arrival at Alpha Centauri.” ( per http://www.aleph.se/Trans/Tech/Space/laser.txt )

    Gliese 581 is 20 LY away, so proportionately greater travel time there.

    I also don’t think that we have the technologies in computer and sensor miniaturization and laser cannons to build this today. If we had an Apollo-level program for a decade, we might be able to launch something like this in 2020 or so, and start getting close-up info on Gliese 581 around the end of the next century – if the probe hasn’t blown a 10-cent fuse, if we still have a working civilization capable of hearing it, etc.

    In science fiction, it’s usually recommended that if you just wait to launch, and keep developing your technology, after a few decades you’ll have a *much* better solution to the problem. (E.g., 1920 didn’t have technology capable of Moon landings, but 1970 did.)

  11. #11 Sean McCorkle
    April 23, 2009

    I’m SO very glad to see the topic of interstellar probes popping up these days!
    for example, Seth Shoshtak in the NY Times recently
    http://www.nytimes.com/2009/04/14/opinion/14shostak.html_r=1&scp=1&sq=shostak&st=cse

    The genius of starting the Apollo program in 1960 was that the Moon mission wasn’t quite possible at that point; a host of problems had to be solved and technologies developed first in order to achieve the goal. We’re still benefitting from the technological progress spurred on by the program (minature electronics, materials etc)

    I would love to see a serious attempt at an interstellar “century” probe for the same reasons, as well as for the pure excitement and promise of discovery, even if I don’t live to see the objective reached. The human race needs to explore.

    and Gliese 581 would be an excellent candidate objective IMHO: four planets so far!

  12. #12 Lab Lemming
    April 23, 2009

    Once you get over 1 Earth mass, gravitational self-compression becomes important, so you can’t just scale up planetary densities.

  13. #13 D_E_R_M_A_N
    April 25, 2009

    Thank you

  14. #14 eddie
    May 7, 2009

    I second Lab Lemming’s comment.
    Obviously the density changes with depth, but also gravity changes. g is zero at the centre as all force balances.
    You’d have to do a calculation where the total mass is the integral, from the centre out, of density as function of radius. You’d then have to refine it with stuff like compressibility of whatever materials you use.
    Start with the assumption of all water and find what radius gets what mass.
    I’m not up to this just now :-(

  15. #15 mike
    July 25, 2009

    Its funny, as we discover earthlike worlds around stars ever closer to Earth, the UFO nuts are going to increasingly come out of the woodwork declaring each world as the homeworld of their favorite alien species based on evidence so scant as to clearly betray them as cultish fruitcakes.

  16. #16 Steve
    December 31, 2009

    Some how in the future if we utilize the power of anti matter for fuel with some advanced space ships and I know this sounds like out of a science fiction movie but Cryo sleep is not fiction, scientists predict we will have cryo sleep by 2040. If all these fall into place we could travel to gliese 581 d as early as 2100. But not to disappoint anyone but there is probaly NOTHING there, from recent satalite images scientists predict it is only a water planet with no land masses.

  17. #17 Anonymous
    June 1, 2010

    A comment on Steve’s comment… (number 16)
    Just because the exoplanet probably has no land masses does NOT mean it cannot sustain life. I do admit, the chances are probably slim to none that there is life there, but not all life has to be like Earth life. Perhaps extremophile life-forms exist on Gliese 581 d. Perhaps they’re nothing more than a few single celled organisms with no mind. Still, it’s the fact that life exists elsewhere that really excites scientists like me.

  18. #18 readup
    July 30, 2010

    what about the Laser light that the SETI Australian detected from there ? If it’s a STARwisp, they should be in the area in about 200 years or so………