I have never heard of such an expression, but i am really in need of such an expression being used more.

Reading several posts and not least spending a lot of time on http://exoplanet.eu/ i've come to miss a new expression. Maybe it already exists, but i am not really sure about this. However, it would make the job easier for a lot of astrobiologists and SETI-folks and much more interesting when looking at the factsheet of a newfound exoplanet on http://exoplanet.eu/ .

OK, we know Earth is located in average 1,49 million km from Sol - this is what we call 1AU. Furthermore, we know that the luminosity of a star is proportional with 1/(r^2). This means that if we move a planet 41% further away from its parent star, the luminosity is be halved.

What not combining these two into a new expression (in case it doesn't exist). Why not make an expression that tells you how much light the planet receieves? Come to think of it, i am pretty sure it does exist, but in that case, why don't we use it more often in exoplanet searching?

To me it will be much more interesting to know how much light the exoplanet receives (ignoring the atmosphere of course). This way we can easily get an overview on how big the chances are for survival? It would be very benefitial for a page like http://exoplanet.eu/ .

In case that expression doesn't exist, or it doesn't exist in this very final definition that i would like to see it in, then i would like to lay claim to it and call it...:

Luminositious Units - LU

Defining: 1LU = 1AU for Earth

Furthermore, it will be easier to find the Green Zone after this, since we don't have to say that with a luminosity twice that of Sol the Green Zone is located at an area 41% further away from the star than our sun, we just simply say that for all star systems the Green Zone are found at ~0,8-1,1LU. Better than using AU, right?

Example:

An exoplanet is found orbiting a star at 4AU and the (star)_lu = 2 x Sol_lu .

So the planet is located at 8LU (correct me if my calculations are wrong ).

So do we have such a definition as i believe we have, and what's the reason for not using it?

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i think the words you're searching for are illuminance (for light) and irradiance (for total energy). In SI units, the former is measured in lux (pronounced "looks") and the latter in watts per square metre.

We call the amount of illuminance and/or irradiance a planet receives from its star the insolation.

Grant Hutchison

Off hand, I find this intriguing. But I'll need to do some maths and physics work before I can comment...

Maybe the solar constant is what you are looking for.
There is a nice table of distance and solar radiation for the planets in our solar system in that wikipedia article.

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Yes, that's the irradiance component. The corresponding illuminance is 130,000 lx.

Grant Hutchison

OK, great, but i would still love to see something easier to compare, like 1AU equals an exoplanet that receives the same amount of energy (of course, not just visible light) as Earth does. This way we can sort the exoplanets by energy and easily compare it to Earth and start talking about which exoplanets we believe could harbour primitive life since the Green Zone is easy sorted out from that lists. Yes, you can do it by looking at the watts aswell, but would be easier if we had another definition of energy received per area unit. Just like it is easier to comprehend / compare an exoplanet that is located at 10AU rather than 1496 million kilometers from its parent star.

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It seems to me the solar constant is exactly what you're looking for. There's no reason you couldn't use it as a unit for the insolation received by other planets (Mars' average insolation is about 600 W/m² or 0.43 solar constants, frex).

One might perhaps wish for a more illustrative name, but it's already better than "astronomical unit"!

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Where I went to school it was pronounced "lucks".

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But i just think that it is quite unfortunate that we don't use it more than what we do. Why don't we use it when talking about Green Zones? What is the needed 'solar constant' for water to exist? I know the albedo on the surface of the planet is interesting to in this talk, but what if we use an albedo like that of Earth? And what if we take the planet in our solar system with the biggest albedo and the smallest albedo, what are the minimum and the maximum solar constants for an exoplanet then to be in the Green Zone?

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I think that AU is more general- you can apply it to, and use it for, many things. Whereas a measuring unit based on Green Zones might be restricted to just that. Maybe.

But i would still find it very useful, espically when planetbiologists and exoplanethunters talk about life.

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Are you trying to reinvent the wheel?

It is commonplace for astronomical publications to express the luminosites of other stars in terms of the Sun, and the distances to companion stars and/or planets in astronomical units. That makes the comparison easy, with nice small numbers. Are you not aware of this?

What I see here is verbally and mathematically incoherent. At some points it is hard to tell whether you are talking about units of light intensity or about units of distance, or perhaps some conglomeration thereof. I cannot tell whether your calculations are right because you did not give an explicit equation for this "location" of the hypothetical planet.

In very short you can say it is an amount of energy per square feet a planet would receive in a given distance RELATIVE to the sun. Yes, we have an expression for energy received by a planet per square feet, so i figured, but not one that is relative to that of the sun. It's like talking about a planets distance from the sun in kilometers instead of using our so beloved AU.

In numbers:
Earth receives 1321 - 1413 W/m^2, and if we set this to 1 of some sort of new unit (the table in this link under the subtitle 'Sunlight intensity in the Solar System' also shows this but doesn't have a name, which i believe it should http://en.wikipedia.org/wiki/Solar_c...Solar_constant), then we could easily compare any other exoplanet's solar constant (which was the word i had forgot) with that of Earth in one single word.

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Sporally, the solar constant applies, by definition, only to the Earth. So you can't talk about the "solar constants" of other planets. However, you can measure the irradiance of other planets in multiples of the solar constant.
Which seems to be exactly what you want.

Grant Hutchison

Measuring radiance is fine and I comprehend the reasons for wanting to. Its not the ideal tool for understanding distances. I like the AU. If I want a larger scale Then its LY. Measuring a stars light output and attaching numbers of luminosity to it measures what? That stars radiance or output. You might need to help me with my understanding of what you are looking for here? please. Grant Hutchison has said it well for me., but have I missed the point here.?

Does solar constant have a symbol? I suggest sc, if it is not already taken, or cs if you prefer a more Gallic flavor¹. 0.4 sc is too little (dim?) for a planet to be habitable, 1.4 sc is too much. Unfortunately it is ambiguous: illuminance or irradiation? They can differ radically for M dwarfs.


1. The Bureau International des Poids et Mesures recommends the symbol "ua" for the astronomical unit, but no-one seems to pay any attention.

Is it? Googling for define: solar constant only finds definitions in terms of irradiance.

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I still cannot see just what it is that you find troublesome. If that Wiki table is not normalized to your satisfaction, you can fix it easily with a few minutes with a calculator. It already gives the distances to the planets in AU.

Start with the equation I = L/R²

where L is the star's luminosity,
R is the planet's orbital radius in astronomical units,
I is the intensity of the star's light as it illuminates the planet.

For this exercise let the Sun's L = 1, and use that as the unit of measure for other stars. This already is a common practice in published tables of stellar luminosity. Were you unaware of this?

This equation will yield I = 1 for Earth at its mean distance of 1 AU.

Suppose we wish to find the "green zone" distance from another star of known luminosity in solar units. Solve the equation for R and set I = 1. The result is R = square root of L. What could be simpler for green zones at a glance?

To fix the Wiki table, just take the inverse squares of the distances in the left column and substitute them for SI values in the right column. This will give Earth's mean value as unity and convert the others accordingly.

If you wish to have a distinctive name other than "solar constant" for Earth's value, try naming it after yourself. Then you will have something analogous to joules, newtons, watts, amperes, ohms, volts, etc. You could shorten it to something like "spor", in the manner of shortening Faraday to "farad" for the standard unit of capacitance.

Grant Hutchison, is there anything you don´t know?

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Yeah, you're right: that seems to be common enough in English these days. I shouldn't be proscriptive about it.
It started life as a Latin word pronounced with a French accent, so "looks" was what I was taught.

Allen's Astrophysical Quantities defines the solar constant as "total solar irradiance = flux of total radiation received outside the Earth's atmosphere per unit area at the mean Sun-Earth distance" and assigns it the symbol "S".

As far as I know, yes.

Grant Hutchison

So true. Grant's a star.

Having taken two years each of french and latin that makes complete sense. :-)

RE: the OP: Maybe instead of creating a separate expression in the SI system, maybe it would be better to create a subsystem that concentrates on maintaining a unit with a specific context. Maybe it could be called the Rare Earth Equivalency Index or something like that, and your unit could be called the REEILux, (pronounced relux) or the LuxREEI, pronounced luxury). Imagine the future papers that would be written about how the conditions for earth-like life are so rare, it's a luxury. It's a rather elegant pun, if I do say so myself.

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@ Hornblower
I might not have been too clear before, but i guess we're on the track now. I was aware of that equation (remembered it when i saw it again) and of course knew it existed though i couldn't remember it. However, i don't think i was aware that if I = L/R^2 , and I equals one for that of the Sun, R equals 1AU for that of Earth, then L would be 1. But of course when i think about it

But to say it short, why not get an expression for this 'Solar constant measured in Earth's average received intensity' so to say. Then we could do a little calculation and saying that "Scientists have now found a planet with 1,03Spor (in case we call it Spor as you suggested)". Then everyone would know how it would be for life to be on that planet - in case the atmosphere was somewhat that of Earth. Moreover, we could get another expression later for 'Solar constant measured in Earth's average received intensity at the surface', but that's another discussion.

With the news of 1,03Spor, everyone would know how well life as we know it could exist there, and it would be very comfortable for scientists talking with the media saying that 'We've now built a new telescope that could find exoplanets at the size of Earth down to 0,8Spor' or something.



Hope you understood me now...

How do you make this R^2 look right as Hornblower did?

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Strictly, as I said above, we would say "Scientists have now found a planet with an irradiance of 1.03 solar constants." Which would be abbreviated as 1.03S.

You use [sup] and [/sup] tags.
So typing "R[sup]2[/sup]" gives you R².

Grant Hutchison

I was going to be a pedant and point out that capital letters should only be used in symbols where the symbol is derived from a person's name (eg Hz), but since the unit is now the Spor I think that objection has been answered.

I think that is an SI convention for units, but it doesn't apply to constants like the solar constant (S). Consider, for instance, the gravitational constant (G) and the gas constant (R). Of more relevance to Sporally's problem, we often measure star masses in units of solar mass (M_) and star radii in solar radii (R_).

Grant Hutchison

Then there's the AU, though I am still unclear why it is usually in caps.

I agree with much that has been said, but this new term should include the stuff that makes it "green". Wattage or Solar constant units alone will not make a planet green (liquid water I assume is the prefered green factor). Albedo, greenhouse gas effects, etc. must be considered to have a green term, right?

It seems to me that the "habitable zone" view has lost some of its impact now that life on warm moons may be possible (ie Enceladus). So it may be harder to sell the term.

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Honorstly, i once said to myself that if i would ever become a famous astronomer some day and event a new expression, i wouldn't name it after myself but find a more related name like for instance the LU (which i started out with). But i find it very funny that a handful of you guys in this debate are already using the term Spor

That is so true, and i am aware of this, however, since we today know nothing about exoplanets in regard to greenhouse gas effects and other things (but probably know more about the Albedo) i would suggest a term for the 'irradiance of solar constants'. But we could surely find new terms where we include more aspects. So... a Geor is when the albedo is taking into account - who will go for the one with the greenhouse gas effect included? OK, maybe getting out of the topic here..



Testing: R²

So now you just write M['sub]something[/sub] to get M_something (found it by quoting your post). Nice!

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And [noparse]M[sub]something[/sub][/noparse] if you want to screen your tags from automatic parsing, so that you can show someone else how the tags work ...

Grant Hutchison

Best to stay clear of a "Geor"; it has been known to stir an offical anal response, of sorts, especially in matters of planetary nomenclature.

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