Cancer Posted March 27, 2007 Report Share Posted March 27, 2007 Several big surveys for detection of extrasolar planets by means of photometry rather than spectroscopy are starting to pay off. Spectroscopy detects planets via the Doppler effect, measuring the reflex velocity of the star caused by the gravity of a planet going around it. Approximately all the extrasolar planets found in the last dozen years have come by this technique. Advantage: you don't have to stare at the star, just observe it every so often; and you can see a bigger fraction of the planets that are out there. Disadvantage: you can only observe one star at a time, and it takes a lot of light to get spectra of the requisite accuracy, which means it takes large telescopes (about 3 meters and up). Photometry is measuring the apparent brightness of an object. To detect planets this way, you stare at a piece of sky and see if any of the stars there get periodically fainter by about 1%, when the planet goes in front of the star and blocks some of the light. Advantage: requires much smaller telescopes (~1 meter is adequate), and you can measure all the stars in a field at once; an also be done efficiently which much fainter stars than the spectroscopic technique. Disadvantage: you only detect those which transit, i.e., the orbit is exactly edge-on to us, which is a much smaller fraction of the planets that are out there. Important point: Only using both techniques can you measure the radius of the planet. With an orbit you get a mass for the planet, and combining mass & radius you get a density, which is a very crude tool toward estimating the overall composition of the planet. (Jupiters have densities like 1 ton per cubic meter; rocky planets are 3 tons per cubic meter and up.) Both techniques strongly favor detection of "hot Jupiters", that is, big massive planets very close to the central stars. Seems that some of those are bigger and fluffier than predicted. Standard models include the effects being warmed by the star, which causes the outer envelope of the planet to expand under the heat, but something else must be going on in some cases. From the April 1 2007 issue of Astrophysical Journal: Only a month ago, HD 209458b was the single known case of a hot Jupiter that is almost certainly too large to be explained by standard models of planetary structure.... With only one strong anomaly [i.e., when only HD209458b was known], explanations requiring somewhat improbable events were perfectly viable. However, now that a significant fraction [4 of 9, including Jupiter and Saturn would boost the 9 to 11] of the transiting hot Jupiters are found to be similarly in need of this additional energy, the burden of the theorists may shift to seeking explanations for this effect that are more generally applicable. These hot Jupiters have radii about 10 to 15 % of the radii of the central stars; the ones that are too big are larger than model predictions by as much as 50% or so. FWIW, these planets that are "too large" are all in orbit around K stars rather than F or G stars; it isn't clear yet whether that's important, or what it might mean if it was important. Quote Link to comment Share on other sites More sharing options...
Old Man Posted March 27, 2007 Report Share Posted March 27, 2007 Re: Exoplanet detection news That could be anything. It could be that K stars emit too weakly and so the oversized planets are able to hoover up more volatiles during accretion. Or tidal forces could be to blame. Or it could be some kind of statistical or spectroscopic error in the measurements that just exaggerates the perceived Doppler effect on K stars. Quote Link to comment Share on other sites More sharing options...
Cancer Posted March 27, 2007 Author Report Share Posted March 27, 2007 Re: Exoplanet detection news I tend to agree with you. I'm pleased that the photometry guys are staring to have discoveries fall out of the pipeline. Those have somewhat different systematic detection effects than the spectroscopists, so we can expect results that disagree over the next few years. And disagreement is where you start learning good stuff. Quote Link to comment Share on other sites More sharing options...
Basil Posted March 28, 2007 Report Share Posted March 28, 2007 Re: Exoplanet detection news That could be anything. It could be that K stars emit too weakly and so the oversized planets are able to hoover up more volatiles during accretion. Or tidal forces could be to blame. Or it could be some kind of statistical or spectroscopic error in the measurements that just exaggerates the perceived Doppler effect on K stars. I'm afraid you misunderstood in what sense the hot Jupiters are "too big." It's not a matter of their masses, but of their radii. Given mass and temperature, the expected radius can be deduced. However, among such hot Jupiters as have had their radii "measured," almost half have radii larger than what the best current models predict. Thus, either the measurements are wrong, or the models are. But if the measurements are wrong, they are so in the same "direction" nearly half the time, which suggests something fairly fundemental is flawed in how the measurements are taken. In either case, interesting times are ahead. Quote Link to comment Share on other sites More sharing options...
Old Man Posted March 28, 2007 Report Share Posted March 28, 2007 Re: Exoplanet detection news I'm afraid you misunderstood in what sense the hot Jupiters are "too big." It's not a matter of their masses' date=' but of their radii. Given mass and temperature, the expected radius can be deduced. However, among such hot Jupiters as have had their radii "measured," almost half have radii larger than what the best current models predict.[/quote'] That's exactly what I meant. Accretion of more, lighter volatiles should create larger planets (as measured by radii) for given mass. Not sure if that would enlarge them to the point that is being observed, though. There's only one way to find out. Quote Link to comment Share on other sites More sharing options...
Cancer Posted April 25, 2007 Author Report Share Posted April 25, 2007 Re: Exoplanet detection news In the paper this morning was an announcement by the Swiss team that they'd found an Earth-class planet in a star's habitable zone. The star is Gliese 581, an M2.5V red dwarf. This star was previously known to have a planet, designated Gl 581b, with a 5.36-day orbital period (assuming the star has a mass of 0.31 solar masses, that turns into an orbital size of 0.041 AU); this planet is inferred to be about the mass of Neptune (15.6 times the mass of Earth). Two new ones were announced this morning. "c" has a mass of 1.6 Earth, an orbital period of 12.91 days, and again assuming the stellar mass that's an orbital size of 0.073 AU. "d" is about 8 Earth masses, period 84.4 days, 0.25 AU. Taking the luminosity of an M3V star and those orbital parameters for "c", then the surface temperature of the planet (assuming it has a surface) is within howling distance of the temperature range for the existance of water in its liquid form. There's lots of "if's" in that conclusion. The technical preprint is at this site. They present solutions for both circular (that is, eccentricity fixed at 0.0)orbits and fit eccentricities; for the last planet in particular the fit eccentricity (0.2 +/- .1) is of marginal significance. The span of observations in the solution is only 1050 days; I'd like to see that doubled or tripled and a better solution obtained. But considering the nature of the planet-finding racket, I'm not surprised they went public with it now. Quote Link to comment Share on other sites More sharing options...
Jaxom Posted April 25, 2007 Report Share Posted April 25, 2007 Re: Exoplanet detection news Interesting... With those numbers I would expect tidal locking for b and maybe c. The numbers don't look like any ratios I would expect though. C and d show a possible ratio of 6 or 7 to 1 which is also not one I recall being stable for resonance. Quote Link to comment Share on other sites More sharing options...
Cancer Posted April 25, 2007 Author Report Share Posted April 25, 2007 Re: Exoplanet detection news Yeah, the strength of the orbital solution for d isn't great. It's the circumstances of c that led to the pressure to take it public. IMO they really need 2-3 times as many data points as they have for good solutions for d, and to firm up c more. Quote Link to comment Share on other sites More sharing options...
Basil Posted April 25, 2007 Report Share Posted April 25, 2007 Re: Exoplanet detection news Boy, and here I thought I'd surprise everyone with this article. Seems Cancer's got much better info. Thanks, Cancer! Quote Link to comment Share on other sites More sharing options...
Outsider Posted April 26, 2007 Report Share Posted April 26, 2007 Re: Exoplanet detection news OK, so spectroscopy detects planets by seeing the wobble they cause in the star. More massive planets = more wobble = easier to detect. Photometry = detecting planets by seeing the star dim as they pass between us and it. Bigger planets = more light blocked = easier to detect. In both cases planets closer to their parent star will be eaiser to detect as both wobble (gravity effect) and light blockage (size effect) willl be more pronounced if the planet is closer. Combining the two one can measure both mass and volume, and thus get density. It doesnt seem to me that the density thus derived would be very accurate, though, being that errors in the two measurements used to calculation it would be compounded. Maybe enough to say "Gas Giant" or "Rocky Planet" but not enough to say "1.5 times earth sized" vs "2 times earth sized". How much of that kind of declaration is guesswork, do you think? basically, how accurate are spectroscopy & photometry in this application? (note : the difference between 1.5 and 2.0 diameters in a 5 earth mass planet means a difference in surface gravity of 2.2Gs vs 1.2Gs...) Quote Link to comment Share on other sites More sharing options...
Cancer Posted April 26, 2007 Author Report Share Posted April 26, 2007 Re: Exoplanet detection news There is no concrete information about the radius of planet c. They don't have photometry; it's purely a spectroscopic result. The stuff in the press about planet radius and size come from the planet mass (which is a lower limit obtained in the spectroscopic orbit solution) and then assuming a density. In our Solar System, the terrestrial planets' densities range from about 3.5 to about 5.5 (the units are g/cm^3 = T/m^3); the Jovian planets are 2ish down to 0.7, IIRC. Known exoplanet densities are limited to hot Jupiters so far, and are even lower still. Presumably that's due to heating "fluffing up" the gaseous outer layers of the jovian planets, though there are problems with that situation. Back to this particular case ... M2.5V stars commonly have spots. I think Gliese 581 is among those known to, in fact. If you have spots on the photosphere, that greatly complicates any attempt to find a planet by eclipses. Quote Link to comment Share on other sites More sharing options...
L. Marcus Posted April 26, 2007 Report Share Posted April 26, 2007 Re: Exoplanet detection news . . . Damn spots! Quote Link to comment Share on other sites More sharing options...
Nyrath Posted April 29, 2007 Report Share Posted April 29, 2007 Re: Exoplanet detection news There is some analysis on the finding here: http://oklo.org/?p=206 http://oklo.org/?p=207 Quote Link to comment Share on other sites More sharing options...
L. Marcus Posted April 29, 2007 Report Share Posted April 29, 2007 Re: Exoplanet detection news . . . So those guys expect a water world? And a transition on the 7th?! Quote Link to comment Share on other sites More sharing options...
Cancer Posted May 11, 2007 Author Report Share Posted May 11, 2007 Re: Exoplanet detection news New item in the Astrophysical Journal yesterday, a (big Jupiter class) planet found orbiting epsilon Tauri. Nothing special about the planet itself: minimum mass of 7.6 Jupiters, 595 day orbital period, orbit size of 1.93 AU. What's special is the host star. It's one of the Hyades giants; the Hyades are a nearby open star cluster about 45 parsecs away (they make up most of the "V" that is the "face" of Taurus) that has been intensely studied for over fifty years. Epsilon Tau is one of four red giants in that cluster, and as such, we know just about all there is to know about it. The cluster is 625 million years old ... relatively young. It's metal rich (this is pretty common for young stars and for stars with planets). And the cluster has been surveyed pretty hard for planets, and none have been found around the dwarfs (main sequence stars) in the cluster. There could be good reasons for that (young stars have more chromospheric activity which makes detecting the velocity variations caused by a planet more difficult to detect), but it's still a little odd to find a planet orbiting a giant. The other that we know is that the star has a mass of about 2.7 solar masses, which is more massive than any of the planetary host stars previously detected. It was (when on the main sequence) an A0 or A1 star, like Vega or Sirius; details of the spectral lines in such stars make it more or less impossible to detect planet-induced velocity changes in such stars; it's not a technology problem, it's intrinsic in the spectra of early A-type stars. So the reason none have been found orbiting such stars before is because it wouldn't be possible to detect them if they were present. So, clear proof that more massive stars can have planets. No one is surprised, but it wasn't proven before. Though arguing from a sample size of 1 is hazardous the detection of a planet orbiting a giant rather than in any of the 100 or so lower-mass main sequence stars in the cluster is a weak suggestion that perhaps more massive stars are more likely to have planets than less massive ones. That's an interesting suggestion ... given that massive stars don't last long, that would suggest there's lots and lots of planets out there orbiting dead white dwarfs or floatign in free space after the massive star died off and lost a lot of mass ... mass loss lowers the gravitational binding energy and can make orbits become unbound. FWIW, the 1.93 AU orbit size is larger than many orbits, but given the star has been through the tip of the red giant branch and is now back on the core helium burning red giant clump, its radius has been as large as about 1AU in the past, and it would have consumed any and all planets that had been within 1 AU of the star, so those in larger orbits are all that would have survived to be detected now. [EDIT: misremembered the stellar mass; it's 2.7 solar masses, not 2.] Quote Link to comment Share on other sites More sharing options...
L. Marcus Posted May 11, 2007 Report Share Posted May 11, 2007 Re: Exoplanet detection news *sits up in bench, pays attention, takes notes and records lecture* Quote Link to comment Share on other sites More sharing options...
Cancer Posted September 29, 2010 Author Report Share Posted September 29, 2010 Re: Exoplanet detection news This might be in the morning newspapers: they've found another couple of planets in the Gliese 581 system, one of them (GJ 581 g) with a mass 3.1 Earth masses, with an approximately circular 36.5-day orbit around an M3V star that is plausibly in the in the habitable zone. I'll be reading the preprint overnight. Quote Link to comment Share on other sites More sharing options...
dmjalund Posted September 29, 2010 Report Share Posted September 29, 2010 Re: Exoplanet detection news you know, "Hot Jupiter" sounds like an exclamation made by a Side Kick in a 1950's Pulp SF comic Quote Link to comment Share on other sites More sharing options...
McCoy Posted September 29, 2010 Report Share Posted September 29, 2010 Re: Exoplanet detection news In the paper this morning was an announcement by the Swiss team that they'd found an Earth-class planet in a star's habitable zone. "Earth-class" in this context means Earth mass (more or less)? Quote Link to comment Share on other sites More sharing options...
Old Man Posted September 30, 2010 Report Share Posted September 30, 2010 Re: Exoplanet detection news 3.1 Earth masses and at the appropriate distance from its star to have liquid water. Whether it actually has any water would require further observation. Gliese 581 is only 20 ly away. Road trip anyone? Quote Link to comment Share on other sites More sharing options...
Lucius Posted September 30, 2010 Report Share Posted September 30, 2010 Re: Exoplanet detection news 3.1 Earth masses and at the appropriate distance from its star to have liquid water. Whether it actually has any water would require further observation. Gliese 581 is only 20 ly away. Road trip anyone? Count me in! Wasn't Susano supposed to be finding us some ships and proper sails? He will find there are men here, too, who do not fear the appalling distance. Lucius Alexander The palindromedary crunches some numbers Quote Link to comment Share on other sites More sharing options...
megaplayboy Posted September 30, 2010 Report Share Posted September 30, 2010 Re: Exoplanet detection news at 1 percent of lightspeed, a mere 2000 years away. Quote Link to comment Share on other sites More sharing options...
IndianaJoe3 Posted September 30, 2010 Report Share Posted September 30, 2010 Re: Exoplanet detection news at 1 percent of lightspeed' date=' a mere 2000 years away.[/quote'] An Orion might be able to get up to 10% of c, so it's only 200 years away. Quote Link to comment Share on other sites More sharing options...
austenandrews Posted September 30, 2010 Report Share Posted September 30, 2010 Re: Exoplanet detection news An Orion might be able to get up to 10% of c' date=' so it's only 200 years away.[/quote'] In a century or two, that should be within the average lifespan. Sweet! Quote Link to comment Share on other sites More sharing options...
Cancer Posted September 30, 2010 Author Report Share Posted September 30, 2010 Re: Exoplanet detection news The GJ 581 primary is M3V, so it'll be rather deficient in ultraviolet. Tanning beds required. The thing with an orbit that close to the star is that the planet almost certainly is tidally locked, so that there's a "day side" and a "night side". Quote Link to comment Share on other sites More sharing options...
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