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The vast majority of the sun's light will never intercept matter again. What fraction reaches the major and dwarf planets? The vast majority of the sun's light will never encounter matter again. What fraction reaches the major and dwarf planets?
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Given the diameter in km D, and the average distance from the sun in AU, the fraction for each object is   Given the diameter in km D, and the average distance from the sun in AU, the fraction intercepted by a round object object is:
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   2.793E-18 * ( D / AU )^2^    (pi*D^2^/4)/(4*pi*(149598000*AU)^2^) = ( 1 / (16 * 149598000^2^)) * ( D / AU )^2^ = 2.793E-18 * ( D / AU )^2^
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|| Planet || Diameter || Distance || Intercepted ||
|| || km || AU || f
raction   ||
|| Planet || Diameter km || Distance AU || Intercepted Fraction ||
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|| Venus || 12104 || 0.72 || 7.89e-10 ||  || Venus || 12104 || 0.72 || 7.89e-10 ||
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|| Jupiter || 142800 || 5.20 || 2.11e-09 ||  || Jupiter || 142800 || 5.20 || 2.11e-09 ||
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|| Ceres || 975 || 2.77 || 3.46e-13 ||       || Ceres || 975 || 2.77 || 3.46e-13 ||
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|| Eris (approx) ||  2700 || 67.67 || 4.45e-15 || || Eris          || ~2700 || 67.67 || 4.45e-15 ||
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So, 99.99999958% of the Sun's light leaves the solar system. Half of the tiny fraction intercepted is by Jupiter, 19% by Venus, 11% by Saturn, 10% by Mercury, and 8% by Earth, with a tiny fraction of the tiny fraction intercepted by the other planets, dwarves, moons, asteroids, and Kuiper belt objects.   So, 99.99999958% of the Sun's light leaves the solar system. Half of the tiny fraction intercepted is by Jupiter, 19% by Venus, 11% by Saturn, 10% by Mercury, and 8% by Earth, with a small fraction of the tiny fraction intercepted by the other planets, dwarf planets, moons, asteroids, and Kuiper belt objects.
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A hypothetical giant telescope mirror 100 meters across and 100 light years away might intercept 1.1E-32 of the Sun's output, about 4 microwatts. At that distance, the Sun would be magnitue 7.6, not quite visible to an unaided human eye. If intelligent life is rare, then chances are nobody would notice if we captured all the light. OTOH, a 50AU diameter infrared sphere would still be visible (and anomalous!) to a very distant astronomer with a large infrared telescope. Perhaps we should be looking for such objects. A hypothetical giant telescope mirror 100 meters across and 100 light years away might intercept 1.1E-32 of the Sun's output, about 4 microwatts. At that distance, the Sun would be magnitue 7.6, not quite visible to an unaided human eye. If intelligent life is rare and widely spaced, then chances are nobody would notice if we captured all the light.

On the other hand, consider a 50AU diameter Dyson shell surrounding our sun, with an average black body temperature of 56K and a peak infrared wavelength of 50 microns by Wein's Law. That would be quite visible (and anomalous!) to a very distant astronomer with a very large, orbiting infrared telescope. An orbiting 1000 meter collector would see a round object (rather than a diffuse cloud) at 10,000 light years; such a collector ( including a triple layer reflector and outside a similar Dyson shell ) might weigh a few tons, an infinitesimal fraction of total shell weight.  Large, sharp-edged infrared spheres may prove to be the most reliable sign of well behaved shell civilizations elsewhere.
Once such civilizations are discovered, narrow beam optical wavelength interstellar communication becomes possible.

Sunlight Intercepted by Planets

The vast majority of the sun's light will never encounter matter again. What fraction reaches the major and dwarf planets?

Given the diameter in km D, and the average distance from the sun in AU, the fraction intercepted by a round object object is:

  • (pi*D2/4)/(4*pi*(149598000*AU)2) = ( 1 / (16 * 1495980002)) * ( D / AU )2 = 2.793E-18 * ( D / AU )2

Planet

Diameter km

Distance AU

Intercepted Fraction

Mercury

4878

0.39

4.37e-10

Venus

12104

0.72

7.89e-10

Earth

12756

1.00

3.15e-10

Mars

6787

1.52

5.57e-11

Jupiter

142800

5.20

2.11e-09

Saturn

120000

9.54

4.42e-10

Uranus

51118

19.18

1.98e-11

Neptune

49528

30.06

7.58e-12

Dwarf Planet

Ceres

975

2.77

3.46e-13

Pluto

2300

39.44

9.50e-15

Eris

~2700

67.67

4.45e-15

Total

4.17e-09

So, 99.99999958% of the Sun's light leaves the solar system. Half of the tiny fraction intercepted is by Jupiter, 19% by Venus, 11% by Saturn, 10% by Mercury, and 8% by Earth, with a small fraction of the tiny fraction intercepted by the other planets, dwarf planets, moons, asteroids, and Kuiper belt objects.

A hypothetical giant telescope mirror 100 meters across and 100 light years away might intercept 1.1E-32 of the Sun's output, about 4 microwatts. At that distance, the Sun would be magnitue 7.6, not quite visible to an unaided human eye. If intelligent life is rare and widely spaced, then chances are nobody would notice if we captured all the light.

On the other hand, consider a 50AU diameter Dyson shell surrounding our sun, with an average black body temperature of 56K and a peak infrared wavelength of 50 microns by Wein's Law. That would be quite visible (and anomalous!) to a very distant astronomer with a very large, orbiting infrared telescope. An orbiting 1000 meter collector would see a round object (rather than a diffuse cloud) at 10,000 light years; such a collector ( including a triple layer reflector and outside a similar Dyson shell ) might weigh a few tons, an infinitesimal fraction of total shell weight. Large, sharp-edged infrared spheres may prove to be the most reliable sign of well behaved shell civilizations elsewhere. Once such civilizations are discovered, narrow beam optical wavelength interstellar communication becomes possible.

SunlightPlanets (last edited 2024-02-18 09:54:00 by KeithLofstrom)