You assume that the orbits of planets today is the same position they've always had. Once some measure of stability occurred the minor changes in mass from the sun would over time would be responded to gently resulting in the positions the planets occupy today.
Nice paraphrase of the standard model crap that comes from your favorite astronomer.
However, if you actually extrapolate this stuff for yourself (it's not hard,) you will find that simply isn't the case.
If the Sun were >= 4.6 billion years old we can calculate the minimum amount of mass lost in the form of electromagnetic radiation(again, neglecting particle radiation and mass ejecta,).
How? By using the Solar constant, to calculate the average power output of the sun via the formula for the area of a sphere with radius = 1au.
Once we have the power, we can convert to total joules of energy produced by the sun over the alleged 4.6 billion years, and then once we have this number, using Einstein's E = MC^2, we can find how much mass the Sun must have LOST in order to produce that much energy.
Now I have done the hard work, and have come up with the number:
http://en.wikipedia.org/wiki/SunCurrent estimate of mass of the entire sun: 1.9891×10^30 kg
Take one electron volt, multiply that times 17million for the energy released in hydrogen fusion, multiply by avogadro's number then divide by two to get energy per gram. Multiply by 1000 to get energy per kilogram of fused hydrogen.
This is: ~8.2x10^14joules per kilogram.
Now Solar constant is 1366 watts per square meter at earth distance from the sun. As stated, I can use this to find the sun's total power output, and consequently it's energy per second, and therefore it's mass loss per second.
distance to sun is ~1.5x10^11 meters
Area of sphere is 4pi X r^2.
area of the sun's sphere of influence at earth distance is 2.827x10^23 meters
Giving total power ouput at: 3.8622x10^26 watts.
multiply this number by seconds, minutes, hours, days, years, for supposedly 4.6 billion years.
5.6x10^43 joules
divide by joules per kilogram shown above for hydrogen that would be lost by the sun over 4.6 billion years at its presentpower levels. (in the past, power levels would have been higher, as there was more mass and more hydrogen, thefore more gravity and more collisions.)...
Anyway:
5.6x10^43 joules / 8.2x10^14joules per kilogram = 6.83x10^28 kg!!!!
using E = MC^2 we can also see how much mass would be lost by the sun. Which is:
6.22x10^26kg in the form of mass converted to EM radiation alone.
In other words about 100 earth masses, or 0.3% of the Sun's mass, would have escaped the Sun as light!! Once again, this does not even consider particle radiation or mass ejecta.
This would also mean thta the sun's average power output would have been at least 1/3 of one percent greater when it first formed than it is now.
Now one third of one percent doesn't look like a big number, but if you removed one third of one percent of the mass of any planet or the sun in the solar system, all of that object's satellite's would immediately begin spiraling outward and escape in relatively short order...
If the sun was this much more massive in the past, then the planets should not have formed at their present orbits and velocities. They should have spiraled into the sun and been incorporated into it's own mass. That is, if the sun was more massive in the past, then the planets would have needed a higher velocity to avoid being sucked in.
On the other hand, if the sun formed with it's present mass and the orbits of the existing planets, then it should have lost 0.3% of it's mass by now, and the planets should have spiraled away.
This gives us a problem much like the whole Dark Matter issue with the formation of galaxies. If the sun really were that old, "Something" would have to offset the loss of mass, otherwise the planets would have all escaped by now, as the average mass lost during that time would have been 0.15% (between 1/6th and 1/7th of one percent.) This is a small number, but it is more than big enough to destroy the entire solar system.