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Looking for "New Earth" in the Alpha Centauri system



 
 
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  #1  
Old October 30th 04, 11:52 AM
AA Institute
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Default 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  
Old November 1st 04, 12:11 PM
Marshall Perrin
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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
  #3  
Old November 2nd 04, 11:23 AM
Thomas D. Ireland
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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]
  #4  
Old November 2nd 04, 11:23 AM
AA Institute
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Posts: n/a
Default

(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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