Some fuss is being made over a newly discovered planet around a red giant star (like here, for example, and on some online conversation I’ve had). Why?
It’s not the star. This is the tenth such red giant star to have a planet found circling it.
It’s not the planet. The planet has a minimum of 4.6 times Jupiter’s mass (that’s big), putting it firmly in the range of previous planets found.
Is it the orbit? Well, yeah. The orbital period is 360 days. And hey, the Earth’s orbit is 365 days. So some folks, not surprisingly, assume that means this planet has an Earthlike orbit!
But that’s only one way to characterize an orbit, and actually it’s not a very useful one.
First, let’s look at the star, called HD 17092. It is an old red giant, and has a mass 2.3 times that of the Sun. That means its gravity is stronger than the Sun’s, of course. The speed at which a planet orbits its star depends on the star’s gravity — in fact, it increases as the square root of the star mass. If the star has four times the Sun’s mass, than a planet at a given distance from that star will orbit it at twice the speed it would if it orbited the Sun (the square root of 4 = 2).
Since HD 17092 has 2.3 times the Sun’s mass, a planet orbiting it at the same distance the Earth is from the Sun will move at a speed square root(2.3) = 1.5 times as quickly as the Earth does. If it were one AU (the Earth-Sun distance) from the star, it would orbit it in just 243 days. Since we know it actually orbits in 360 days, it must be farther from the star. If you do the math, you find that the planet is actually 1.3 AU from its star. So it’s actually 30% farther out from its star than the Earth is from the Sun. That’s kinda sorta Earthlike. Maybe.
You might think that would make the planet colder, but wait! We need to look at two more things.
The star is a red giant, which means it’s cooler than the Sun. Its temperature is about 4600 Kelvin, compared to the Sun at 5500 Kelvin. Again, you’d think this would mean the planet would be cooler than the Earth. But (still) wait!
The star is a giant. That means it’s bigger than the Sun, and in this case HD 17092 is about 10 times the diameter of the Sun. That means a planet orbiting it will receive more heat from it at a given distance. I have the equations written out if you care to look, but the bottom line is that the temperature of a planet compared to the Earth’s depends on the temperature of the star and the square root of the star’s size. When you do the math (and I hope I did it correctly), you get that the temperature of the planet is about 2.3 times that of the Earth (assuming lots of things like: it has the same reflectivity as Earth, it rotates rapidly, and other stuff that will affect this calculation, but probably not by much).
So actually, the planet’s surface will be pretty hot: roughly 500 Celsius, or 900 Fahrenheit. That would melt lead and zinc! Remember, though, the planet has more than 4 times Jupiter’s mass, or well over 1000 times Earth’s mass, so it’s probably a gas giant with no real surface at all.
Oh — the orbit of the planet is more elliptical than Earth’s orbit (eccentricity = 0.17 versus 0.09 0.02).
So. The planet is whoppingly huge; it’s boiling hot; it has a star that appears 8 times bigger in its sky (the width of three fingers held at arm’s length), but is much redder, than the Sun; it orbits 1.3 times farther out from its star than Earth does the Sun; and its orbit is more oval than Earth’s.
But hey, at least the period is 360 days! That’s Earthlike!
Mind you, I’m not making fun of people who were saying that (except for anyone who would write a headline saying it) because we’re not used to thinking in terms of temperatures, distances, square roots, and stellar sizes. We hear the number 360 and that sounds familiar. But in every way that counts, the orbit isn’t Earthlike at all.
This is an alien world, orbiting an alien star. There will come a time — and it will be soon, I guarantee it — when we will find an Earthlike planet orbiting a Sunlike star in an actual Earthlike orbit. And that, my friends, will truly be a day. And I’ll be right here to talk about it.
August 3rd, 2007 at 12:15 pm
Hmm. Large planet orbiting a giant red star…
Holy crap, it must be Krypton!
August 3rd, 2007 at 12:16 pm
Some years ago I read an article in which a gene engr. discovered that if the ends of a DNA strand were sealed(instead of open, as in normal earthly DNA) that the DNA was stable and biochemically active at 750 degrees F. I can’t for the life of me recall where I read this. Must have been before I got this computer(with it’s 80GB drive. Hey that’s almost infinite, right?) but my personal memory is usually right,,,anyway, the implication was that with such molecular stability, life could possibly survive and thrive on a very hot planet, with other environmental considerations appropriate, (ie, that particular DNA still requires H2O for it’s functionality).
Thus I wonder if such gas giants could have complex life forms? Maybe some day, we’ll know.
Gary 7
August 3rd, 2007 at 12:51 pm
A good example of how “minor” differences between stars and planets quickly add up to very major ones. Especially when speculating about relatively fragile, niche systems (in the sense that it requires a very specific range of conditions) as Earthly life.
Complex life on gas giants (and possibly elsewhere; Venus-like worlds and so on); I’d love for life to be found to be that widespread. Won’t know until we get out there, ourselves or our robot proxies, and do some extensive poking about…
August 3rd, 2007 at 1:16 pm
[...] also this Bad Astronomy blog entry. __________________ Science is a way of trying not to fool yourself. The first principle is that [...]
August 3rd, 2007 at 1:17 pm
An equally interesting question is whether conditions on the planet would have been more “Earth-like” when the star was on the main sequence, before it evolved to become a red giant. The star would have had a surface temperature hotter than the Sun (about 11,000K), and have a larger diameter (about 1.7 that of the Sun).
With those numbers it appears that the temperature would have been similar to now (if I have done the math right). Except of course, stars lose lots of mass in the giant phase, so planets will migrate outwards. How much mass has this lost?
August 3rd, 2007 at 1:50 pm
Just as a suggestion: when you post equations, it might be easier to render them in LaTeX and take a printscreen of that and just embed the image. It’d certainly be easier to read. You can do a “poor man’s” version of this by writing up the equations in the Wikipedia sandbox (using the <math> tag) and saving the PNG that it renders it to. PNG renders text better than JPG, you don’t get the fuzziness …..
But I agree. Gliese was much more interesting. Hopefully this comment will render the escape characters for less-than and greater-than …
August 3rd, 2007 at 2:05 pm
“There will come a time — and it will be soon, I guarantee it — when we will find an Earthlike planet orbiting a Sunlike star in an actual Earthlike orbit. And that, my friends, will truly be a day. And I’ll be right here to talk about it.”
Phil, to your knowledge has there ever been a plan of events for when this occurs? Are we leaning towards unmanned probes? Generation ships? Cryogenic freezing? Earth 2 colony ships?
Granted – once we get the FTL, artificial gravity, and shields problems fixed, this will all be a moot point….but still….anyone wanna take a crack at this?
August 3rd, 2007 at 2:42 pm
The mass is about 2.3 times solar, which suggests that when it was on the main sequence it was of spectral type A, with properties similar to those of Fomalhaut (also around 2.3 times the solar mass), which has a luminosity about 16 times greater than that of the Sun. With a semimajor axis of 1.29 AU around such a star, the planet would have received a flux of about 10 times the power per unit area that the Earth does. A naive temperature estimate would put the planet at around 500 K (200 C), but that doesn’t take into account greenhouse effect, non-blackbody properties, internal heating, etc.
August 3rd, 2007 at 2:46 pm
Oh btw, a summary of the planet properties (and the preprint of the discovery paper) is available here.
August 3rd, 2007 at 6:44 pm
I was just thinking along similar lines Bob. Finding an habitable planet outside our solar system is great news, if we can just manage getting there.
I have no doubt they exist, just like I have no doubt a large chunk of expensive rock that would set me up for life exists within a 10 km radius of my house under ground somewhere.
I am more excited about a future prospect of a habitat on ‘Earth like’ planets in our own solar system, just like I am more excited about earning money the hard but sure way of working hard, and earning money so I can by lots of stuff. ( I could however dig up all of the dirt under my house and find that rock, yeah right)
Mars of course comes to mind, maybe a moon or 2 around some of the giants (yes some aren’t planets and all are not truly Earth like), but Ill be happy if we can just set up a base on the moon first.
Of course, finding the elusive ‘Earth like’ planet will enforce the idea of life on other worlds, but for me, thats already a given. To think other wise is a silly idea in my mind, given all the evidence at hand. Sure it would be nice to find it for ‘proofs’ sake, but meh, lets get excited about what we can work with first, namely our solar system.
We can actually travel to these places.
August 3rd, 2007 at 7:58 pm
Once we discover a planet that appears to actually be Earthlike due to the orbital, planetary, and stellar properties, then you can bet that NASA will get funding to build a telescope that can image it. We have ideas on this, and you don’t need an overwhelmingly large telescope to do it, just some advanced tech.
As far as going there? Nah. That’ll be awhile; we don’t have the tech to do it in under 1000 years. An advanced ion drive might work, but I don’t know how fast they can go over a distance of, say 10 light years. But anyway, that’s many, many decades off, and we have a whole solar system to explore first!
August 3rd, 2007 at 8:15 pm
I was talking to a friend at work the other day about this very subject and we were wondering something. There are quite a few stars in our local neighborhood, say within 20ly or so. We hear about all the planets being found around stars much farther away but I have never heard about anyone studying the stars in out local group. Have all these stars been eliminated as planet-bearing? Im asking because if we find planets around other stars and some day want to travel to them, it would be nice if they were a little closer to home, if you know what I mean.
BA, do you have any data on this question?
August 3rd, 2007 at 10:30 pm
Simply put Bob there is no FTL as far as we know. I doubt very much there will ever be any sort of FTL. I refuse to even debate the possibility of FTL travel at this point.
A spaceship operating under conventional relativistic laws using advanced ion drive or plasma drive like VaSIMR would still need something on the order of a billion tons of fuel to cover the distance to another star. Even then it would take something like 1000 years. We can’t even build power plants that last for much more than fifty years yet.
I think the Bad Astronomer is being optimistic at putting interstellar travel on the order of 1000 years. These days I put it on the order of tens of thousands of years to hundreds of thousands of years if ever. Homosapien might be long gone by then and replaced by a much more space friendly form of intelligent life.
August 3rd, 2007 at 11:34 pm
[...] Earthlike orbit? Charts on Fire. Laser Printers Cause Cancer. How Would You Change the Zune? Anti-Drunk Driving Tech-equipped Nissan. Giant Russian Tetris Game. New Virgin Airlines Planes. [...]
August 3rd, 2007 at 11:35 pm
RayCeeYa,
I respectfully disagree. Not with your assessment of FTL possibilities, but with your timeline to interstellar travel. Humans have, in a few thousand generations, gone from colonising Europe, Asia, and the Americas in a time on the order of 10000 years, to today being able to move between any two points on Earth in a time measured in days. I think that it is realistic to say that, given sufficient impetus, our travel time to the Moon or Mars can be measured in years (and after the revival of the necessary programs and technologies, that time will concievably be down to months).
My point is simply that nobody alive today can possibly imagine what even fifty years into the future may hold. To borrow a metaphor from Richard Rhodes, in less than fifty years, the idea of atomic power went from Rutherford’s criticism of it as “moonshine”, to the Trinity test reflecting its light off of the moon.
August 4th, 2007 at 1:36 am
Nitpick: The Earth’s eccentricity is 0.016, not 0.09.
And as for interstellar travel, we have the technology to do it today (albeit slowly) using NSWR propulsion (http://www.npl.washington.edu/AV/altvw56.html). That should get to around ~5% of C; this star is ~400 LY away, so it would take around 8,000 years.
August 4th, 2007 at 1:42 am
http://img472.imageshack.us/img472/1138/screenshotiv8.png
Hmm…Phil, I see what you’re saying about Digg now…hehe.
August 4th, 2007 at 1:54 am
Well autumn you can disagree if you like, after all my degree is in chemistry not rocket science but as I under stand it, the highest specific impulse thought achievable at this point is with a photon rocket. That is a rocket that expels photons to generate thrust. Photons having a mass of zero and an “exhaust” velocity of the speed of light imply that they would be the the most efficient way for a space craft to generate thrust from it’s fuel.
The problem is they consume massive amounts of energy to generate usable thrust. In the case of a space ship occupied by humans 1g of acceleration is optimal for space travel. That way you simulate earth’s gravity as you accelerate over the incredibly long periods of time. A photon rocket requires something on the order of 300 megawatts per newton of force generated. That’s a little less than 3000 megawatts to accelerate 1kg at 1g. If you multiply that out to a year of acceleration and then go through the mass energy conversion you find that for each kilogram of mass you want to accelerate you have to convert one kilogram of matter to pure energy over the course of a year. Bells should be ringing here because if it takes 1kg of fuel to accelerate 1kg then where’s the engine, or the rest of the ship for that matter.
Current and future technology like fusion require too much fuel to even accelerate at 1g, so we are left with speculative technology like antimatter propulsion. I won’t get into that because I don’t know enough about the handling storage or creation of antimatter to speculate on what he energy cost back here on earth would be making the stuff.
You can go ahead and check my math. I’d be surprised if my back of the envelope 1:00 am calculation wasn’t off by at least a factor of ten. But as I said I’m a chemist not a rocket scientist.
August 4th, 2007 at 9:03 am
D’oh! I knew 0.09 seemed high. Turns out I misread the table, and used Mars’ eccentricity.
Thanks Tom!
August 4th, 2007 at 1:16 pm
Even if a planet is a gas giant, it will still have moons around it, as in our own solar system. Like Titan, in orbit around Saturn, some may be able to retain a respectable atmosphere. However, HD 17092 has temperatures more like Venus than Earth, and that is not factoring in an atmosphere.
August 4th, 2007 at 2:11 pm
Not to mention the radiation levels coming off the star. Nor even the planet’s own radiation.
August 4th, 2007 at 2:11 pm
Thank you for this, I was wondering while reading the articles you mentionned how a red giant star could be like Earth.
August 4th, 2007 at 4:00 pm
Venus? Venus has NOTHING on HD 17092b: doing the maths I obtain a stellar luminosity of 43 times solar for the giant star, which means the planet receives a flux 26 times greater than Earth… by comparison, Venus receives a mere 1.9 times the flux, and Mercury at perihelion 11 times.
Hmmm… that figure for Mercury is quite close to my estimate for the flux received by HD 17092b when the star was on the main sequence. Not a good place for a small moon to retain an atmosphere, plus you’d get a lot of heating in the exosphere because of the strong UV output of an A-type star.
August 4th, 2007 at 4:54 pm
Aldebaran seems to be a similar star. Similar mass and spectral class. HD 17092 is a K0III with 2.3 solar masses and Aldebaran is K5III with 2.5 solar masses. Aldebaran has a longer radius, so maybe they are not so simialar. Just looking for a comparable star. Aldebaran is the best comparison I could find in 5 minutes. Also Aldebaran is closer to us and much brighter (both appearent and absolute).
August 4th, 2007 at 5:15 pm
Aldebaran seems rather more luminous than HD 17092, so may well be at a different stage in its evolution… bear in mind that the giant stage corresponds to very large changes in the star’s properties, and there are several different stages in the lifetime of a red giant. There is no simple mass/luminosity relationship like there is for main sequence stars: an explanation of what goes on in the life of a red giant is given here.
August 4th, 2007 at 7:46 pm
RayCeeYa:
Photon propulsion IS the highest theoretical specific impulse possible however(don’t you just love caveats?), all we really need is an external power source, as in large, solar powered lasers in close solar orbit, to impact a large solar sail. This has been pretty well researched and could result in very high velocities before the craft is too far from the sun to receive the laser. As far as slowing down at the target, one proposal has a small lead craft, loaded with AI, self replicating robots, and an anti matter, photon drive to slow down, with attendant solar sails for close in(well, close enough for the stars light to be significant) deceleration change, sent on ahead to build lasers near the target sun and decelerate the (large) incoming vessel. All this is theoretically possible today, except for the practical problem of (cost effectively) generating enough anti matter for the advance probe.
However, I expect if we start building large numbers of space colonies, we’ll be kept way too busy playing in our own solar system to be settling other solar systems, at least in the near term(read 10,000 years or so) and what kind of tech we’ll have by then is anyones guess. I really expect, if the space colony with fusion power is adequately developed, we’ll have settlers in the Oort clouds all the way to the nearer stars, long before we have any need for FTL.
Last I heard, the Oort clouds extend nearly to the Centaurus Oort clouds.
Anyone wish to correct me on that?
Gary 7
August 5th, 2007 at 9:42 pm
Any sign of other exoplanets or an exoplanetary system around this orange giant star then?
Seems similar to Pollux’s exoplanet “Polydeuces” * … that had a 590 day orbit (3 Jove mass) which I’d mis-calculatedas putting it between Mars & Earth distance based on exactly the “Earth-like orbit error” discussed here. D’oh!
Fomalhaut is type A3, Sirius A1-0 (?) from top of head.
Thought Sirius was twice solar mass but could be 2 & 1/2.
So if the latter tehngoing by what was written earlier Aldebarran used tobe (during its main-sequence H-burning lifespan) a star like Sirius (well Sirisu A that is!)
Cool – thanks for that info. (mass Aldebarran etc ..)
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* Polydeuces -alternate name for Pollux, mythological Greek form
August 6th, 2007 at 1:12 am
Home now with the references handy so can confirm :
Sirius A spectral type = A1 mass “about twice” our Suns, its luminosity is 23 times our Sun’s and its surface temp 9400 degrees Kelvin.
Sirius B (“the Pup” to “the Dogstar”) is an archetypal DA hydrogen-rich white dwarf is about as massive as our Sun – but considerably smaller, (Earth-sized) fainter and hotter.
Fomal haut is an A3 star and thus similar to Sirius but a fraction less massive, smaller and cooler.
Contradictadorily enough, Vega is an A0 type Sirian* dwarf star but confusingly is listed as being only 1.5 times the solar mass yet 54 times as bright versus the respective figures for Sirius A …
(Sources : James B. Kaler, “The 100 Greatest Stars”, Copernicus Books, 2002 & ‘Astronomy’ magazine Stars poster April 2006.)
Incidentally, not to be too pedantic but shouldn’t we call these red giants ORANGE giants instead seeing they are K-type stars?
Surely : O type main-sequence stars = blue dwarfs
B type ” ” ” ” ” = blue-white dwarfs (or just blue?)
A type ” ” ” ” ” = Sirian* dwarfs
F type ” ” ” ” ” = Procyonese* dwarfs
G type ” ” ” ” ” = yellow dwarfs (or solar?)
K type ” ” ” ” ” = orange dwarfs
M type ” ” ” ” ” = red dwarfs
with the same rule applying for giants & supergiants as for dwarfs … ?
Therefore surely the K type Pollux, Aldebaran, Arcturus, etc Plus HD 17092 are actually all Orange rather than Red giants – a shade off but still .. Or do K & M giants just get lumped together as red regardless of spectral colour type?
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‘Sirian’ = main sequence A type dwarf star as calling these stars “white dwarfs” risks confusing them with the very different type of stars -or rather ex-stars such as Sirius _B_ . Ditto “Procyonese”
August 6th, 2007 at 4:00 am
Could you please use a larger font Phil, I can’t read this size.
Sandy.
August 6th, 2007 at 2:45 pm
SCR: I’ve usually seen the spectral class colours given as:
O/B – blue
A – white
F – yellow-white
G – yellow
K – orange
M – red
But for A-type stars and hotter the dwarfs/giants distinction becomes rather less clear or useful, plus even main sequence (“dwarf”) stars are getting pretty big by this point. The term “blue dwarfs” seems to more often refer to a type of galaxy rather than a type of star. Main sequence stars of spectral type A are “white main sequence stars” I guess.
Furthermore these colours are rather approximate, and set up relative to Vega (an A0 star). If you choose your colour system so that sunlight is “white”, M and K are orangey-pink, G is white, F is whitish with a hint of blue, and A onwards get progressively bluer.
Regarding the mass of Vega, you seem to have found a source giving a rather low estimate – the usual figures I’ve seen are somewhere in the region of 2.5 solar masses: SolStation gives 2.3-3.1 solar masses.
As for “red giants”, yes this term does include types K and M. As far as I can tell, there are as yet no known exoplanets around M giants.
August 12th, 2007 at 10:17 pm
Andy – cheers for that.
My source for Vega’s mass was James B. Kaler’s book – ‘The 100 Greatest Stars’ – it does seem low esp. given Vega is often described as quite blueish … But Kaler is the expert in this area so .. interesting,. estimates can vary certainly.
“White dwarfs” would seem confusing to me given the application to both white dwarf stars (about 1 to 1 & 1/2 solar masses jammed down into roughly earth-sized spheres) which are stellar remmants – the old cores of medium mass stars and (about 2 solaramss, many times the solar diameter) A0-type main sequence stars. By the logic above both Sirius A & Sirius B would be white dwarfs yet they are VERY different types of star! Same for F-type yellow-white hence using Sirian & Procyonese to avoid that possible confusion …
March 1st, 2008 at 11:18 am
With regards to Gary 7′s comment about space colonies. I’m not an astronomer or an engineer, but it seems to me that if we ever achieve the technology to build a self-sufficient space colony that is not dependent on regular shipments of supplies from earth, assuming that the colony can support a population large enough not to run into trouble with inbreeding, all you’d have to do is strap some kind of engine to it, and no matter how weak or slow or primitive that engine would be, you’d automatically have a generational starship that could go anywhere in the universe (given enough time) and deliver a functional population to the destination. This would be assuming that said colony/starship was not dependent on harvesting any solar system specific resources, like water from Oort comets, though if the Centaurus Oort cloud and the Solar Oort clouds really are very close together, that shouldn’t be a problem, at least for a trip to the Centauri system.
The only caveat to the above I can think of is that having that kind of technology to build completely self-sufficient and mobile colonies would also mean that we would have no need for earth like planets at all, just raw materials in space to make more colonies, and thus would not have as much incentive to make such journeys in the first place.
Also, if the trip is sufficiently long, the population of the colony could diverge evolutionarily and perhaps wouldn’t be considered human any more, at least not by today’s admittedly limited standards.