Extrasolar Planet Discussion - Now Discussing: KEPLER announces hundreds of candidates

This topic has come up, tangentially, in multiple threads now, and as there's interest but not really a proper place, I figured I would give this a go and see what sort of interest we have in a thread on this, particularly as it's a dynamic field and so there is a lot of opportunity for speculation that doesn't require a strong background on it, as well as discussing the significance of the discoveries being made. Rather than start with a subtopic, I thought I would give an overview of some of the basics first, and then see where the discussion goes from there.


Extrasolar Planets (ESP) are, as the name rather indicates, planets outside our solar system. The objects themselves are rather simply objects that are not massive enough to be stars, although there's not presently a lower constraint for objects outside our solar system as to what constitutes a planet. Additionally, all those that have been confirmed are orbiting other stars, although it is possible and likely that there are some that are free floating, although these have not yet been found.

Currently, we know of just over 450 ESPs. The primary way that they've been detected (around 350) has been by measuring the radial velocities of host stars. A planet orbiting a star causes the star to wobble back and forth slightly, and so by measuring how fast the star is moving as it heads towards us and away from us, we can get a constraint on the mass of the planet, as well as figure out some values for its orbit. We find that most of the objects we've found this way are around the mass of Jupiter to several times the mass of Jupiter, and with orbits that tend to be only several days long. The second most common detection method is to look for planets that go in front of their star as they orbit, resulting in the star periodically dimming as the planet passes in front of it. This allows us figure out, again, the orbit of the planet, as well as the radius of the planet. When we use this method with the above, it means that we can get both the radius and the mass of the planet, which allows for a much more complete picture. We do find that most of these planets through this method are, again, planets around the mass of Jupiter or higher, and that have orbits that are only a few days long, meaning that these planets are much closer than even Mercury is to our own sun. At least one key factor in why most planets we find have these characteristics, though, is that very massive planets with very short orbits are the easiest to find, and so we do expect the characteristics of the population of known ESP to change over the coming years as we develop better accuracy in our techniques and have more time to observe.

That gives a very basic overview of what most of the planets we've found are like and how we're finding them, and I'll leave this at that to then see what areas have the greatest interest to continue onward in.

 

What I'd like to know is this:
We're here, right.
[image=http://jenbayne.files.wordpress.com/2009/10/milky_way4.jpg]
Where are the exoplanets you describe? Is the search being done systematically or randomly? Is there a concentrated effort to like first search the Alpha Centauri system and then go outward, or is everybody looking at their own thing?

 

As much as I love hearing and learning about new planets, the fact that the majority are Pegasean means that they don't really capture the publics attention. It won't be until we not only have the technology to detect, but photograph these planets in detail that the publics imagination will be captured.

Plus, y'know, everyone knows that Pulsar planets are where it's at...

 

Well, there's several different search methods being used, and so different systems are being used.

The first extrasolar planets discovered, actually, were found around pulsars, which are dead stars that rotate very fast, hence the pulse part of the name. The discovery came in 1992, and while it was the first proof of extrasolar planets, it wasn't really what we were after since dead stars weren't what people had in mind.

The first confirmed planets around earthlike stars came from the radial velocity method, which I believe, has simply been going through monitoring periods of stars that are bright. The reason for this is that the brighter the star, the easier it is to get high quality data on the star. A very faint star will be more effected by random noise than a very bright star. I'm less certain on the specific selection methods for candidates using radial velocity measurements, although one of the big things is Sun-like stars. With this method, you have to select specific stars and check.

With the transiting method, it has become far more productive to cast a wider net, and use telescopes that are imaging a large number of stars at once and then look for any that are periodically dimming, then following them up with specific focus if a star looks promising. For example, the Kepler mission, which was launched last year by NASA, is fixed on only one area of the sky where it can see 145,000 main sequence stars (main sequence stars are the normal life of a star, and is what our sun currently is) and is constantly monitoring them. This past January, the Kepler mission announced it's first 5 planets. The other transiting planet searches, such as the ones that are earth-based, have been tending toward the same attitude, which is to look at a large area of the sky, and have computers search the images to find stars that are dimming, and then follow up candidate stars with more focused observations and measurements to verify that the dimming is being caused by a planet. Again, there's a focus for earthlike planets and it's easiest with nearby stars, and each search has their own way of picking where to look.

Alpha Centauri (a multiple star system, with capital letters denoting the individual stars) has been under constant scrutiny, simply as being so close is very convenient, and Alpha Centauri A and B are both very similar to the sun. Planets haven't been ruled out, but because it's been under constant observation, we can rule out very large planets in fairly close orbits at this point, but we can't yet tell if there are earthlike planets or not.

Back to the other part, where we are in the galaxy, I do need to introduce some numbers to get a bit of scale going. Because of the distances involved, we use light years to have more manageable numbers, where one light year is the distance light travels in one year, and is about 10 trillion kilometers or 6 trillion miles. The Alpha Centauri system, containing the 3 closest stars to our solar system, is a bit over 4 light years. The distance from us to the center of the galaxy is around 25,000 light years, and the galaxy, as a whole, is around 100,000 light years across. With only a handful of exceptions, all the extrasolar planets are within about 600 lightyears of us, so we're looking at a very small segment of the milky way galaxy. For a bit of an analogy, if you think of the Milky Way galaxy as the size of the United States, virtually all the planets we know of could be contained within the island of Manhattan in New York City. Again, this really comes down to that we're trying to look at sun-like stars, and so we do have a bit of a limitation on how far they can be from us for us to still get meaningful data.

(If anyone wants me to cover and terms and concepts I skip over, by all means ask. I'll try to make sure I explain any that are directly relevant to where I'm going, but some may slip by, and I don't mind going into greater depth on some of the things that are tangential concepts)

 

As I've posted elsewhere, I believe this is some of the most important science going on today. That shift in my lifetime from "soon, we will find planets orbiting other stars" to "here are some planets orbiting other stars" is nice confirmation to a layperson that astronomers and physicists have their acts together.

Some questions: what are the current technological limits to how far out we can look for planets, how long will it take to catalog planets within those current limits?

What is the next level of technology needed to look farther out or in greater detail?

 

Haven't we only discovered planetary disks(planets forming) and gas giants(big enough to detect) thus far?

Don't we have to have some technological progress in order to detect Earth-like planets?

edit

I have to throw in a conspiracy thing too:

What about Planet X? Are these types of rogue bodies even possible(outside of comets and such)?

 

So... we're in the Mid Rim. Could have been better, but I suppose it could have been worse. Being a Core World would have been nice.

 

Technological limits are a bit hard to nail down - it really depends on what your looking for. Finding planets the most popular way - maping changes in the host star's radial velocity, as Lowbacca already described - comes down to three variables at the telescope: luminosity of the parent star, the mass of the planet, and the separation between the two.

If you want to find worlds that are like Earth, you will, presumably, want to look for them around stars like ours. Our Sun, a G2 main sequence star, has an absolute magnitude (brightness at 10 parsecs) of about 5.0 mag. To get a decent spectra on a star requires collecting a lot of light in an exposure, which usually means the use of a large telescope, long exposure times, or both. If you figure a magnitude limit for doing this kind of science as something like 20 magnitudes (very, very faint indeed, even for the world's largest telescopes) that would put a star like our Sun at a distance of 10,000 parsecs, or a bit over 30,000 lightyears. [EDIT] These are extreme upper limit values for how far we can look to do this work. As Lowbacca said above, we have only been able to go out to about 600 lightyears so far, likely due to limitations on how faint the objects we are studying can practically be. Our Galaxy is about 100,000 light years across, so right now we only have the ability to detect planets in a small percentage of our own galaxy.

The mass of the planet you are trying to detect is also very important, as it is what determines the magnitude in the shift of the parent star's radial velocity. The separation between the star and the planet determines how long the planet takes the planet to complete an orbit. Jupiter, which is a few hundred times the mass of Earth, causes a radial velocity shift in our Sun that is much larger that the Earth's, which explains why the vast majority of exo-planets we have found are so large: they are simply easier to detect. The distance between the two bodies, and thus the length of an orbit, determines how long we have to monitor a system before we can say a planet has been detected. This length of time is typically 2 full orbits of the planet, i.e. 2 years of monitoring to detect a planet that is 1AU away from it's parent star (like Earth) or 24 years for Jupiter.

As to how long it would take to look for planets around every star in the local universe we can... well, that could take quite a while. Even assuming that getting the necessary observing time on a telescope is not a problem (it is, and a big one) we would have to monitor a given system for several years or decades (or centuries, if you want to find things as far out as Neptune) to be sure we got most of the major planets.

 

joeyryanastro Covered a lot on the tech limits, and the constraints we have from the star-planet system itself. Here is a plot that tries to visualise the idea a bit more:
[image=http://www.ast.obs-mip.fr/users/pfouque/IMG/Keith_exoplanets.gif]
It's a few years old, which means it has, in some regards, changed dramatically, but you can see that most of the planets are high mass, shortish orbits. Each of the contours represents a different program, and the colours represent different detection methods. The lower blue line on there is the Kepler mission, which should be announcing earth-like planets in about 2-3 years if it finds them, as it's been up for a year now. It has the capacity to see the small amount of dimming an earth-sized planet will cause, it just needs the time to see an earth-like orbit happen a few times.

We've discovered some smaller planets, but not many. Aside from the pulsar planets, with are around the mass of the earth, we have several planets around red dwarfs that are what are called super-Earths, larger than the earth but less than 10 Earth masses. These have actually been found close to the habitable zone for their stars. I believe the current smallest that is a terrestrial planet around a sunlike star is CoRoT-7b, which is about 5 Earth-masses and appears to be made up primarily of rock, although it orbits extremely close to it's host star.

And the Planet X part... rogue bodies are entirely possible, and so that there could be larger objects as part of our system is something that COULD happen, however it's more that if it were there, we should see some indication of it's effects. It is also fully possible to get planetary bodies that are moving independently, since we do think that there are stages in the early system that open the door to gravitational interactions between planets forcing a planet out of the solar system.

I'll also add on here, now that it's up and running again, a very useful site called The Extrasolar Planets Encyclopaedia
http://exoplanet.eu
It is intended for those in the field, so parts of it are very technical, but it does help to get an idea on what some of the parameters are of the objects we're finding.

 

Ok thx. Now remind me how large a Astronomical Unit is? I used to love my telescope watching as a kid and used to know this stuff but now......blank.

 

An astronomical unit is the distance from the earth to the sun, which is around 93 million miles or 150 million kilometers.

 

Isn't Mars like 2 or 3 AUs from the Sun or something?(to compare how much further it is from us)

 

Mars is around 1.5 AU, Mercury is at around .4 AU. A large portion of ESP are closer than that.

 

I like that you menitoned NASA's Kepler probe, which has been described as the Earth's best chance to find another Earth.

As mentioned, the Kepler probe uses what is called "winking" technology, or the "transit method" in that it detects the "wink" in a star's light as any planets that orbit around it passes in front of it. ("front" being from the probe's perspective.)

The good thing about the Kepler probe is that it is able to use its own computers to remove the human-tedious nature of any search. I believe that Kepler is able to process 100,000 stars per every focus of its field of vision.

The bad news about Kepler is that it can only detect "winks" in stars that are aligned with Kepler's "eye." Stars that have both Earth sized planets and are aligned are projected to be about 1% of those that we can observe. But still, 1% of 100 billion stars in the Milky Way leave A LOT of stars that can be detected by Kepler.

The really, really exciting aspect about Kepler is that NASA expects to find about 50 Earth-sized planets over the course of its mission (along with a lot more non-Earth planets)and NASA also predicts that 12% of stars detected will have multiple planets that orbit them.

I just hope that there's a follow on project that tries to make contact with the planets discovered. The 8 planets that Kepler has either discovered or confirmed in the Cygnus and Lyra systems sit at an average distance of about 11.4 light years away, which is not a horrible distance to send a message and wait for the reply.

 

Kepler's far from the first search that has been computer intensive. Several of the transit searches use the same methodology of searching a field and then having a computer search through to find candidates. What is fairly new is that it's looking at just one area of the sky for the mission run, and even the COROT mission, which is the other mission looking for transiting ESP from a space telescope, only spends 150 days at a time on it's longest areas of sky. Kepler's mission, though, is designed for a much longer run of time on that area of the sky.

And I'd really not give KEPLER confirm credits on TrES-2, HAT-P-7, or HAT-P-11. Those were all confirmed prior to KEPLER's launch, although I'd not be surprised KEPLER's improved the data on them. I would agree that KEPLER is likely the best chance we've got running right now, although I'd also point out that there is a limit to possible stars it could detect. Most stars in the galaxy are simply too faint to get any data from, even with KEPLER. Near as I can tell, KEPLER's limit is 17th magnitude, which really isn't that faint compared to other telescopes.

 

Correct me if I'm wrong though, as I just delve into this stuff because I find it interesting, but COROT wasn't designed to find inhabitable planets, but to simply catalog all the planets it could detect. Whereas Kepler was designed to find "other Earths," if possible. It's just that so far, Kepler has found gas giants just like COROT, as those all the most common type. I still hope Kepler reaches its projected goal to find 50 other Earths, because even just one would have the potential to have dinosaurs or amoebas or other life.

Speaking of computers and searching ability, I read that NASA's upcoming James Webb "Hubble 2" space telescope, with its giant gold plated reflector, has a analog/digital processor that typically would weigh about 9kg (about 20lbs) but its been miniaturized down to the a size that's slightly larger than a quarter.

 

Well, the largest difference is that COROT is looking around at various areas of the sky, so it's a bit harder to find a long orbit, whereas KEPLER's constant monitoring is meant to catch that. COROT still has found a planet with an orbit of 95 days (COROT-9), and can detect at least down to a diameter of 1.7 Earths (COROT-7). So from a sensitivity standpoint, COROT could, in theory, find an earth-like planet in terms of physical characteristics. That they move around in the sky so much means they're not so likely to find planets that have earth-like orbits.

So the biggest differences are just that KEPLER is fixed on one area of the sky, and KEPLER has a much bigger field of vision, COROT apparently is looking at around 12,000 stars, whereas KEPLER can watch a bit more than 10 times that.
So I'd say COROT has the ability to detect an earth-like planet in terms of both physical composition and orbital characteristics, but KEPLER's got better odds on it. I hope KEPLER beats them to it, as if COROT-7 was any indication, COROT is going to be REALLY annoying if they find an earth-like planet in an earth-like orbit. Because they were unbearable when it came to COROT-7, since that was the smallest planet found (in terms of radius) at that time.

 

Nanotech! Or at least Minitech if not full blown nanotech.

 

I think that Kepler folks will be pretty unbearable too if - hopefully when - they catch their first Earth-size planet on an Earth-like distance from it's star. And they will have every right to be.
(And humility in bringing out results has never produced continued funding for science projects anyway. )

But one thing about Kepler is that it's going for quantity, not for planets that are easy to study by future space telescopes and the new big Earth telescopes like E-ELT. It might find literally thousands of planets, but on average, they will be thousands of lightyears away, which will be somewhat a disappointment to many. The now infinitely postponed TPF flotilla and Darwin are/were intended to image Earth-like planets from stars up only to 50 lightyears away. Kepler's planets will likely be "just" points of light for centuries to come, not seen as images where continents, oceans and clouds could be seen.

I would guess that either COROT or telescopes on Earth will find the first (well, first which everyone can agree to be) Earth-size planets on a Goldilocks-zone around red dwarfs, and Kepler will produce the first Venuses, Earths and Marses around stars like our Sun. Search progresses rapidly and Kepler needs few years to find these planets, unlike COROT and Earth based telescopes to find red dwarf Earths.

But, this is a wonderful time to be alive. I feel a bit disappointed - but enlightened too, considering the implications on how the past scientific revolutions were seen by public at large - how relatively little notice the answer to this old question about the existence of planets around other stars has got. I often meet well-educated, intelligent people who have no knowledge about the discoveries (and who often mistake astronomy and astrology...) and then bore them by telling that this discovery is one of the very few things about our age that most educated laymen will know a thousand years in the future. And then this discovery will be presented as a tumoltous change in humanity's view of the universe and our place in it.

 

My comments on CoRoT were very much on a direct people basis. The attitude of at least one of the CoRoT guys is fairly annoying.

I don't expect the same attitude from KEPLER, offhand, and I'd also add that i think the stuff they're finding will be a bit closer.... under a thousand light years but still on the order of hundreds.
We do have a bit of a bias in that, while red dwarfs could well have the right conditions for an earthlike planet, we keep looking only at stars like the sun, which does mean further targets since most stars near us are red dwarfs, not sunlike stars.

 

http://blogs.discovermagazine.com/badastronomy/2010/05/20/star-om-nom-nom-planet-aieee/
New story I'd figure I'd mention here.
The link goes to a blog post that's already a simplified form of it.

The very simple thing is that we've now got evidence that the planet is in the process of being consumed by it's star. This is also a step in understanding this as it's one of a few extrasolar planets that have much larger radiuses than the masses indicate they should have. In this case, at the time of discovery WASP-12 was the hottest planet we'd yet found. This finding keeps WASP-12 as being one of the more intesting planets out there, although it is a Jupiter-like planet, not earthlike, and is in a very close orbit to it's star.

 

How exactly does a star "consume" a planet? Is it just falling into the star?

 

When a star depletes its supply of hydrogen, it starts burning heavier fuels. As it does that, it swells to what we call red giant stars. When the Sun becomes a red giant, it will consume Mercury and Venus. Earth very likely would be scorched, but wouldn't exactly be consumed.

(I can't confirm whether a star's gravity intensifies over its lifetime, so this might not be accurate) Planets which are already in solar orbit would be pulled into lower orbit around the sun if its gravity intensifies when it becomes a red giant. The more intense the gravity, the higher the escape velocity is needed to maintain solar orbit. The slower a body is traveling around a celestial object, the lower its orbit will be.

 

Actually, for what's happening, you have to keep in mind that this is a planet that is similar to Jupiter. So what's going on more is that as the planet is heated, the atmosphere (which is really most of the planet) is expanded. It's also orbiting close enough that the planet is being stretched by the star's gravity. Both of these are resulting in portions of the atmosphere being far enough from the planet that they reach an area where the gravity of the star is strong than the gravity from the planet, meaning that the star is pulling off portions of the planet's atmosphere.
If your classes have covered it, this is somewhat analogous to how when a star reaches a red giant phase, a companion star may gain material from it because the red giant has expanded enough that it's outer layers face more gravity from the companion star than the star it's part of.

This part isn't actually related to WASP-12 consuming WASP-12b, the planet, since WASP-12 is a Sunlike star still. It's about the same mass and size as the sun.

This isn't true. There isn't any mass change to the star (at least not any significant change, although it is technically decreasing in mass over time, although may have some gains from objects hitting it) and the gravity won't change significantly unless the mass is significantly.

Absent from the rest, this isn't actually true. The further a planet is from the sun, the slower it orbits. There's more complicated factors involved, in terms of cause and effect, and really, it presumes stable orbits, but the speed is then determined by the distance. The further out you are, the slower the object moves. Precisely, the orbital velocity is proportional to 1/r^1/2, where r is the radius of the orbit. So as the radius increases, the velocity decreases.

 

Clearly I didn't know what I was writing about. I mistook specific orbital energy for velocity, which was not nearly so simple to understand. And given as orbital mechanics apparently have nothing to do with the subject of the article, I withdraw my last statements. I'll be sure to check my logic before posting on the subject again.