One of the most striking features of our solar system is how flat it is. All the planets (now that Pluto is no longer a member) orbit the Sun in the same plane; if you could see the solar system "from the side" all the planets would look like they fall in a line.
What could cause this? Astronomers centuries ago came up with some theories (for example, a passing star ripped matter away from the Sun which formed planets in a plane– but why then does the Sun rotate in that same plane?) but they didn’t work out. Finally, it was theorized that the Sun and the planets all formed from a flat disk, collecting junk and growing.
This disk theory has become a virtual fact in astronomy over the years, with all sorts of indirect evidence to support it. But the direct evidence was lacking. You’d need to see a planet that is sitting in the disk of material around it, and that’s hard to find. We see lots of disks surrounding stars, but the stars are too young to have formed planets. We see gaps in some disks, evidence of planetary disturbances, but we can’t see the actual planets so we can’t be sure they orbit in the same plane as the disk. And where we see planets well enough to determine their orbital tilt, the stars have long since blown away their disks.
That’s all changed now. Astronomers using the Hubble Fine Guidance Sensors — extremely high-resolution instruments usually used to lock Hubble onto target, but can also be used to observe bright stars — have watched the star Epsilon Eridani for several years. They have seen the star wobble over that time, indicating a planet is there.
The planet has been known for some time. But the extremely nice data generated by the FGSs allows a lot of information to be gathered about the planet. For example, they have found that the period of the planet is 6.85 ± 0.03 years, the semi-major axis (the orbit is elliptical, so this is the long radius of the ellipse) is 1.88 ± 0.20 milli-arcseconds on the sky (this is an incredibly small number, roughly equal to seeing a car on the Moon or a U.S. quarter 2500 kilometers away), and the tilt of the orbit is 30.1 ± 3.8 degrees.
But wait! Eps Eri has a disk (it’s the picture above, from data announced in 2002)… and it turns out the tilt of the planet’s orbit is consistent with that of the disk.
Pretty cool. It’s not proof that the theory is correct, but it’s suggestive. Had the planet’s orbital tilt not been consistent with the disk tilt, the theory would have been in trouble. As it is, it strongly supports the theory.
I’ll note that many young earth creationists like to say that astronomers don’t know how solar systems form, and we’ve never seen planets forming. That’s baloney. I’ve seen that claim on many creationist sites, even long after we’ve seen disks around other stars, and all the other supporting evidence. Think they’ll take that statement down from their sites now? Yeah, me neither.
A few more neat things about this. Eps Eri is pretty close, just 10.5 light years away (a bit more than twice as far as the nearest known star to us). It’s a bit lighter weight than the Sun, with about 0.83 times the mass, and is slightly cooler and redder. The planet is about 1.5 times the mass of Jupiter, and its orbit takes it as close to Eps Eri as the Earth is to the Sun, and out as far as Jupiter (incidentally, the FGS data indicate the possible presence of another planet with a 50 - 100 year period that remains otherwise undetected as yet). When the planet is closer in to the star it gets brighter, and easier to see. Astronomers are hoping to be able to actually get an image of the planet when it next is at periapsis (closest approach to the star), which is in early 2007. This is a very tough observation, since the planet will be so close to the star that the star’s light will drown it out. The separation is only going to be .3 arcseconds, which strains even Hubble’s ability to split them.
Years ago, when I worked on Hubble, we wondered if we could spot Jupiter if it were orbiting Alpha Centauri. I worked really hard on a simulation to see how our camera could observe that, and found in the end we just barely might be able to see it. But I didn’t think we could positively identify it. We never tried; it would take too much time and effort for too low a chance of payoff. We still don’t know if there’s a planet around the nearest known star, which is kinda funny: we know a lot about a planet more than twice as far away.
I thank Dr. Fritz Benedict, who led the team who did this work, for the use of the diagram above.
October 9th, 2006 at 10:55 pm
so does that mean that if a eath-like planet was orbiting alpha centauri, we wouldn’t be able to tell it was there?
or are there other ways of inferring its existence?
October 10th, 2006 at 12:25 am
The Terrestrial Planet Finder may detect an earth-like planet (or two) in the Alpha Centauri system.
Some time ago I wrote up a little computer program to see how a planet’s orbit would behave in the system. It appeared that earth-like planets could remain in the “Habitable Zones”, although the eccentricities varied significantly in cycles.
October 10th, 2006 at 1:06 am
“You’d need to see a planet that is sitting in the disk of material around it, and that’s hard to find.”
Especially since, technically, it wouldnt’ be a planet. Woot.
October 10th, 2006 at 2:02 am
Needless to say, a cool discovery!
By the way, Epsilon Eridani is not the only planetary system with a known disk–a few others systems are known to have one. However, it seems to be the only nearby star with a planet in a relatively distant orbit.
incidentally, the FGS data indicate the possible presence of another planet with a 50 - 100 year period that remains otherwise undetected as yet
Actually, the existence of second planet was first (?) infrerred from the shape of the dust disk. Some further evidence comes from the radial velocity measurements. However, its orbital period is far too long to be confirmed with radial velocity measurements.
We still don’t know if there’s a planet around the nearest known star, which is kinda funny: we know a lot about a planet more than twice as far away.
I think the most amazing thing is that we don’t know if Alpha Centauri system has any planets but we know that there is a frozen giant terrestrial planet orbiting a red dwarf star 21,000 light years away!
October 10th, 2006 at 2:18 am
so does that mean that if a eath-like planet was orbiting alpha centauri, we wouldn’t be able to tell it was there?
Right.
BUT if we build a telescope that stares only Alpha Centauri B and measures its radial velocities for a few years collecting tens of thousands of data points, we could soon know if there is a terrestrial planet with the mass of Mars orbiting in the habitable zone!
For more, see http://oklo.org/?p=117 and some later blog entries.
How much would it cost? Well, the recent discovery of planets in the galactic bulge required seven days of Hubble observations, in other words it cost millions of dollars. A telescope like this could be cheaper.
October 10th, 2006 at 4:43 am
I wonder about that habitable zone.. I’m no expert at all, but with a star as big as Alpha Centauri I see problems with the proximity required to get enough light and warmth when compared to the amount of radiation and other hazards of living near that star.. also, although potentially less of
a problem, I suspect that this habitable zone might be so close that a planet orbiting at that distance might have a tidal lock with the star. I’m thinking along the lines that a star becomes cooler and outputs less light the redder it gets, and you have to get closer to get more heat and light.. I might be thinking along the wrong lines however..
October 10th, 2006 at 5:16 am
Pandabear,
You’re thinking of Proxima Centauri (Alpha Centauri C), a red dwarf star. The star under discussion is Alpha Centauri A, a type G0 main sequence star slightly larger, hotter and brighter than the Sun.
October 10th, 2006 at 5:59 am
I see! I thought that redder meant less bright and less hot since a flame changes color and brightness as it’s heat changes too and thus red stars should be colder than our yellow star in general regardless of mass.. obviously there’s other factors that matter here.. thanks!
October 10th, 2006 at 6:52 am
The star under discussion is Alpha Centauri A, a type G0 main sequence star slightly larger, hotter and brighter than the Sun.
The star under discussion is Alpha Centauri B, a type K0 main sequence star slightly smaller, cooler and dimmer than the Sun.
I do not know why B instead of A, but I guess B may be chromospherically quieter.
Since Proxima is likely a member of the system, it has the same metallicity as the two larger stars and therefore is a good candidate for having planets.
October 10th, 2006 at 7:05 am
Pandabear, I think you’re confused between the members in the Alpha Centauri system: Alpha Cen A is a yellow (class G) star like the Sun, Alpha Cen B is orange and slightly cooler (class K star like Epsilon Eridani) and Proxima (a.k.a. Alpha Cen C) is a red dwarf, cool and dim.
The only reason for this is the mass. Alpha Cen A is slightly more massive than the Sun, which is slightly more massive than Alpha Cen B, which in turn is much more massive than Proxima.
A’s habitable zone (HZ) is larger than Sun’s and B’s HZ smaller; it is interesting that both HZs are stable, so both stars can have habitable planets.
Proxima’s habitable zone is indeed small and very close to the star. Nearby, strong stellar flares and the fact that a closely orbiting planet turns same face towards its star may be problematic in the case of Proxima, but some studies suggest that even these factors don’t necessarily make a planet unhabitable.
October 10th, 2006 at 7:26 am
BA, is that the same Fritz Benedict of the U of Texas? If so, I know him. We went to high school together about 40 years ago,,,
GAry 7
October 10th, 2006 at 11:17 am
Doesn’t the habitability of a planet depend greatly on whether or not it has an atmosphere conducive to the life forms which dwell on the surface, which need an atmosphere to protect them from the cosmic radiation? Perhaps warmth from a sun isn’t the greatest need?
October 10th, 2006 at 11:38 am
A certain minimum of heat is necessary to have an atmosphere. Though this could feasibly be geothermic, that would not be practical to test at this distance, wheras stellar radiation is.
October 10th, 2006 at 3:08 pm
Well of course Epsilon Eridani has a planet. In fact, it’s got at least 3. There’s a Great Machine inside one of them and maybe the ancestors of a bunch of brothers named Zathras that collects all sorts of information about the local galaxy.
October 10th, 2006 at 3:33 pm
Heh I was about to post something about Babylon 5 but I got beat to it.
October 10th, 2006 at 4:14 pm
Very cool.
I did not know the disk theory didn’t have direct evidence of it.
Well, does now.
At least no creationists have come over to argue their case against this, like last time.
October 10th, 2006 at 6:21 pm
I find it weird that we don’t know more about Alpha Centauri. I would think it would be the easiest system on which to gather data, and the most logical, considering we’re not likely to develop an instantaneous stardrive before we have some kind of interim, high-energy method of insterstellar travel: we’d be going there first, if anywhere.
October 10th, 2006 at 9:15 pm
[…] Phil Plait, the Bad Astronomer, has a cool post up that’s just out of this world – literally. Phil discusses the planet orbiting Epsilon Eridani. […]
October 11th, 2006 at 2:29 am
I always thought Alpha centauri was a red supergiant.. now where did I get that idea?
October 11th, 2006 at 11:44 am
The BA sez: “1.88 ± 0.20 milli-arcseconds (this is an incredibly small number, roughly equal to seeing a car on the Moon)”
A car on the moon? Didn’t we leave three “buggies” up there from Apollos 15, 16 and 17? Could the FGS be used to image them, like the HiRISE images of the rover on Mars last week? The Russian’s have a couple of similar-sized Lunikods as well. The LM descent stages are larger still.
- Jack
October 11th, 2006 at 2:35 pm
Pandabear: I always thought Alpha centauri was a red supergiant.. now where did I get that idea?
Have you ever played a computer game called Star Control II (Accolade, 1992)? It depicts Alpha Centauri as a red supergiant. It’s astronomy is mostly the Bad variety… However, I still remember myself laughing for gas giants in the game that had temperatures over 1000 kelvins. Such planets were considered impossible–but that was before 51 Pegasi b!
Alpha Centauri is the third brightest star in the sky. The only reason for this is that it is so darn close! Put it at the distance of, say, 20 light years and it wouldn’t be hardly notable: It would be slightly brighter than its analogue Eta Cassiopeiae, a star between the second and third star as counted from right in Cassiopeia’s W.
Jack Hagerty: Could the FGS be used to image them, like the HiRISE images of the rover on Mars last week?
No way. Firstly, FGSs are not cameras. Secondly, Hubble can’t resolve objects smaller than 100 m (IIRC) from the surface of the Moon because it is so distant. The orbital distance of Mars Reconnaissance Orbiter from Mars was less than 300 km when it photographed Opportunity. That is more than 1000 times closer.
October 11th, 2006 at 10:57 pm
I eat a dick everyday.
October 12th, 2006 at 4:22 am
Kullat Nunu: Hell yeah! IMHO one of the most brilliant SciFi games ever! The astronomy there might be horrible, but Fwiffo’s dialogue when you find him on Pluto makes me lose bladder control every time
By the way, there is by the way a remake of SCII available on Sourceforge somewhere which plays more or less the same, and there’s voices with it.. do a search on The Ur-Quan masters and you should have no problem finding it..
A game that should’ve known better is David Braben’s Frontier (and First Encounters for that matter) which actually tries to be accurate in astronomy when dealing with interplanetary physics.. Alpha Centauri is a red supergiant there too..
October 12th, 2006 at 4:28 am
Fundamental Error number 1.
“We still don’t know if there’s a planet around the nearest known star, which is kinda funny:”
There are eight of them, now that Pluto’s been relegated…..
October 12th, 2006 at 10:05 am
[…] And then, we shall ask the question again: Are we living in a Flat World (Earth)? Of course not, once the Earth is seen as a “whole” - in a larger scale. But, wait, what if we increase the scale of our purview to a scale that includes the Solar System in its entirety. Now, are we living in a Flat World? Or perhaps in a Disk World? Phil Plait at the Bad Astronomy blog explains more and discusses the recent insights usign the Hubble… […]
October 12th, 2006 at 4:22 pm
Kullat Nunu sez: “Firstly, FGSs are not cameras. Secondly, Hubble can’t resolve objects smaller than 100 m (IIRC) from the surface of the Moon because it is so distant. The orbital distance of Mars Reconnaissance Orbiter from Mars was less than 300 km when it photographed Opportunity. That is more than 1000 times closer. ”
Thanks for the response. I am aware of the various pieces of hardware you mention, and their capabilities and missions. What I was responding to (and apparently misunderstanding) was Phil’s statement in the post that the pixilated image shown was somehow generated with the FGS and that it was resolving at that incredible level. If it was measuring redshift or some other spectrum changes to generate that image, than obviously it wouldn’t work on an object sitting on the moon since the object and the background are moving together. What caught my eye, obviously, was the analogy to a car on the moon, which there actually are several of.
- Jack
PS - Sorry about ending that last sentence with a preposition…
July 19th, 2007 at 11:42 am
[…] has been validated many times, and astronomers now accept it as true (I’ve written about this here, here, and here, for example). In fact, we’ve gotten pretty good at finding young stars still […]