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Looking for "New Earth" in the Alpha Centauri system
"New Earth" is just my favourite way of referring to a hypothetical
planet orbiting around either one of the two principal stars in the Alpha Centauri system, within their respective *habitable* zones. So... assuming such a planet does exist, I'd like to know if we could hope to see it *directly* through a telescope using special techniques like masking out the glare of the star itself. Focusing on Alpha Centauri 'A', which is of spectral type G2V - exactly like the Sun - and whose habitable zone is located at between 1.2 to 1.3 AUs out, is there a projected *magnitude* that an Earth-sized planet is expected to have? If indeed it exists, at maximum elongation from the star New Earth would wander a total of 2.7 arc-seconds out and be amply within astrometric resolution limits for even amateur sized telescopes! Since *resolution* is not an issue and *masking* out the star itself in a telescope is no problems, then clearly how bright such a planet is expected to be will be the determining factor for its successful detection. I know Hubble has detected objects down to as low as 30th magnitude in the realm of faint galaxies, so I'm wondering if New Earth is expected to be of a magnitude figure brighter than 30th magnitude and if the Hubble was able to mask out the brilliance of Alpha Centauri 'A', whether or not theoretically, it would have been able to image suach a planet directly. I am also wondering if there's any projections for the expected magnitude of a Jupiter-sized planet orbiting within the habitable zones around each star. Why bother with a gas giant orbiting within the habitable zone? Because New Earth could be one of the *moons* of such a giant planet! Any thoughts on the expected magnitudes of such exo-planets, anyone? Abdul Ahad |
#2
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AA Institute wrote:
"New Earth" is just my favourite way of referring to a hypothetical planet orbiting around either one of the two principal stars in the Alpha Centauri system, within their respective *habitable* zones. So... assuming such a planet does exist, I'd like to know if we could hope to see it *directly* through a telescope using special techniques like masking out the glare of the star itself. There are a number of research projects underway right now towards this problem. It's actually an extremely difficult technical challenge, but one which a lot of people are very excited about. See: Gemini Extreme AO Coronagraph http://www.gemini.edu/science/aspen/...-announce.html VLT Planet Finder http://www.eso.org/instruments/vlt2ndgenins/ Terrestrial Planet Finder http://planetquest.jpl.nasa.gov/TPF/tpf_index.html Focusing on Alpha Centauri 'A', which is of spectral type G2V - exactly like the Sun - and whose habitable zone is located at between 1.2 to 1.3 AUs out, is there a projected *magnitude* that an Earth-sized planet is expected to have? In visible light, a terrestrial planet is around a billion times fainter than its parent star - so around 15th magnitude for Alpha Cen A. Planets, at least by themselves, would be fairly trivial targets for many telescopes these days. But don't get too excited just yet... If indeed it exists, at maximum elongation from the star New Earth would wander a total of 2.7 arc-seconds out and be amply within astrometric resolution limits for even amateur sized telescopes! Since *resolution* is not an issue and *masking* out the star itself in a telescope is no problems, i Here, alas, you are mistaken! Masking out the star is in fact a daunting technical challenge. Because of the wave nature of light, it will diffract around any obscuration that you put in front of it. It's easy to mask out *most* of the starlight with a simple hard-edged black spot and matching Lyot pupil - say 99% or so. That still leaves the residual starlight 10^7 times brighter than your hypothetical planet! Blocking the starlight to the requisite degree (99.9999999%!) requires very fancy optics and exquisite control of wavefront aberrations. Various optical systems have been proposed to do this - coronagraphs, apodized pupils, shaped pupils or cat's-eye masks, pupil remapping apodizers, nulling interferometers, achromatic nulling interfero-coronagraphs, even a thousand-mile long pinhole camera in space! There's really quite an impressive zoo of proposed designs these days, each with their various pros and cons. *All* of them incorporate adaptive optics systems substantially beyond the current state of the art, in order to correct wavefront distortions that would otherwise completely prevent the starlight from being blocked successfully. It's a really, really hard problem. I am also wondering if there's any projections for the expected magnitude of a Jupiter-sized planet orbiting within the habitable zones around each star. Why bother with a gas giant orbiting within the habitable zone? Because New Earth could be one of the *moons* of such a giant planet! In reflected starlight, a Jupiter will be around 100x brighter than an Earth at the same distance from the star (that's just the ratio of their areas, assuming roughly equal albedos). However, a *young* Jupiter (under a billion years) can be much brighter in the infrared, as it's still radiating away its original heat of formation. How much brighter? As much as a thousand times or so, resulting in a contrast ratio of "only" 10^4 or 10^5. Thus young Jupiters are potentially the easiest exoplanets to image directly, a fact not lost on the people designing AO planet finders for Gemini and the VLT. - Marshall |
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AA Institute ) wrote:
: "New Earth" is just my favourite way of referring to a hypothetical : planet orbiting around either one of the two principal stars in the : Alpha Centauri system, within their respective *habitable* zones. : So... assuming such a planet does exist, I'd like to know if we could : hope to see it *directly* through a telescope using special techniques : like masking out the glare of the star itself. I think that if there was a planet around Alpha Centauri or Proxima Centauri that we would have heard about it long before this. Having said that, I does make for an interesting research and development metaphor. A large jupiter sized planet would or could be like winning the jackpot in a lottery where there could be multiple earthlike moons. Tom [Mod. note: top-posting fixed. please don't top-post -- mjh] |
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(Marshall Perrin) wrote in message ...
AA Institute wrote: "New Earth" is just my favourite way of referring to a hypothetical planet orbiting around either one of the two principal stars in the Alpha Centauri system, within their respective *habitable* zones. So... assuming such a planet does exist, I'd like to know if we could hope to see it *directly* through a telescope using special techniques like masking out the glare of the star itself. There are a number of research projects underway right now towards this problem. It's actually an extremely difficult technical challenge, but one which a lot of people are very excited about. See: Gemini Extreme AO Coronagraph http://www.gemini.edu/science/aspen/...-announce.html VLT Planet Finder http://www.eso.org/instruments/vlt2ndgenins/ Terrestrial Planet Finder http://planetquest.jpl.nasa.gov/TPF/tpf_index.html Thanks. I wasn't aware of the first two links you supplied, although I've heard of the TPF. Do you have any ideas as to how far we are from the TPF mission actually being in orbit and operational? Not too many decades I hope... Why can't a mission like this be prioritised, I find its goals extremely worthwhile. Focusing on Alpha Centauri 'A', which is of spectral type G2V - exactly like the Sun - and whose habitable zone is located at between 1.2 to 1.3 AUs out, is there a projected *magnitude* that an Earth-sized planet is expected to have? In visible light, a terrestrial planet is around a billion times fainter than its parent star - so around 15th magnitude for Alpha Cen A. Planets, at least by themselves, would be fairly trivial targets for many telescopes these days. But don't get too excited just yet... I've managed to patch together a basic magnitude model that lets you project the expected visual (V-band) magnitude for an extrasolar planet (exoplanet) having Earth- or Jupiter-like photometric properties, which is located in the habitable zone and shining by the light reflected from its parent star, as viewed from our vantage point here on Earth. In theory, the model should hold accurate for gauging the brightness of an exoplanet around *any* star, since by the default definition of a "habitable zone", a planet would experience a total light flux of a constant planet/star ratio in all cases. m2 = 5/2 * Log10 R + m1 [where m2 = magnitude of planet, R = star/planet brightness ratio (Earth = 1,499,684,836, Jupiter = 9,120,108) m1 = apparent visual magnitude of parent star ] My article in full, is he- http://uk.geocities.com/aa_spaceagen...r-planets.html This model is of course only focussed on what matters most: a planet located in the *habitable zone* where a "New Earth" must surely exist! I'd be keen to hear any views/disagreements about my assumptions underlying this model and the validity of its results. If indeed it exists, at maximum elongation from the star New Earth would wander a total of 2.7 Correction - that's a mistake on my part...the max. angular separation of an Earth-like planet located in a circular orbit within the outer edge of the habitable zone around Alpha Cen 'A' should be: Arctan (1.3/272,000 AUs) = 0.986 arc-seconds, not 2.7! arc-seconds out and be amply within astrometric resolution limits for even amateur sized telescopes! Since *resolution* is not an issue and *masking* out the star itself in a telescope is no problems, i Here, alas, you are mistaken! Masking out the star is in fact a daunting technical challenge. Because of the wave nature of light, it will diffract around any obscuration that you put in front of it. It's easy to mask out *most* of the starlight with a simple hard-edged black spot and matching Lyot pupil - say 99% or so. That still leaves the residual starlight 10^7 times brighter than your hypothetical planet! Blocking the starlight to the requisite degree (99.9999999%!) requires very fancy optics and exquisite control of wavefront aberrations. Various optical systems have been proposed to do this - coronagraphs, apodized pupils, shaped pupils or cat's-eye masks, pupil remapping apodizers, nulling interferometers, achromatic nulling interfero-coronagraphs, even a thousand-mile long pinhole camera in space! Gosh! Tell me no more! I hadn't realise how much effort is already underway in this area and the true scale of the technical challenge involved. Nice to see so much focus here though, as I'm sure the returns would be highly worthwhile. In my view, a hunt for a "New Earth" around nearby stars is far more worthwhile than trying to decide if the Coronae Borealis supercluster of galaxies is larger than the Hercules supercluster... Surely, it doesn't matter too much in the grand scheme of *us* on our *lonely* little world trying to branch out into a few nearby stars in our humble corner of the vast Milky Way galaxy?! There's really quite an impressive zoo of proposed designs these days, each with their various pros and cons. *All* of them incorporate adaptive optics systems substantially beyond the current state of the art, in order to correct wavefront distortions that would otherwise completely prevent the starlight from being blocked successfully. It's a really, really hard problem. I am also wondering if there's any projections for the expected magnitude of a Jupiter-sized planet orbiting within the habitable zones around each star. Why bother with a gas giant orbiting within the habitable zone? Because New Earth could be one of the *moons* of such a giant planet! In reflected starlight, a Jupiter will be around 100x brighter than an Earth at the same distance from the star (that's just the ratio of their areas, assuming roughly equal albedos). However, a *young* Jupiter (under a billion years) can be much brighter in the infrared, as it's still radiating away its original heat of formation. How much brighter? As much as a thousand times or so, resulting in a contrast ratio of "only" 10^4 or 10^5. Thus young Jupiters are potentially the easiest exoplanets to image directly, a fact not lost on the people designing AO planet finders for Gemini and the VLT. A *young* Jupiter implies it will be too young to have a sufficiently stable, mature *moon* around it - which would render my own ambitious quest for a "New Earth" located in the habitable zone - fruitless... Thanks Abdul Ahad |
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