Planets based on size?

Question

Lately I have been thinking a lot about planets and how large do they need to be, not to mention how far does planet need to be in order for it to be like Earth. Mainly I want to know how far and how big a planet need to be to be habitable, especially if the star is as big as the one called Sirius, or nearly as big. I heard today that the Earth's equator is about 24,901.55 miles, and the Sun's diameter is equal to 109 Earth's)

Answer 1

The question you ask is a bit complicated. It may help if you refine it. This will help put "bounds" on possible answers.

What is an "inhabitable" planet. Inhabitable by what? We, of course, tend to think of "life as we know it", meaning that the planet has to have features that would be OK for the kind of life we know (oxygen for animals, nitrogen for plants, sufficient gravity to retain a dense atmosphere, yet not too much gravity otherwise bones will break...).

You also write "in order for it to be like Earth". Of course, if we ask you "How much like Earth do you want it to be", then your answer to that question will be the answer to your original question.

As for placing an Earth-like planet around Sirius, what are you mostly concerned about:

That the planet has the same temperature as Earth? That the planet receives the same amount of light as Earth? That the planet has no more dangerous radiation as Earth (UltraViolet, X-ray, gamma rays from Sirius)?

Each of these will have a different effect on the distance at which your planet has to be placed. And each distance may have an impact on what kind of planet (size, mass) you need to retain the atmosphere (a hot atmosphere escapes to space faster than a cool one), and that may have an impact on the type of life that can grow there (gravity depends on size and mass).

Earth is 150 million kilometres from the Sun. Our Sun has a radius of 696,265 km and Earth's equatorial radius is 6,378.14 km. Thus the Sun's radius is 109.16 times that of Earth.

This means that the surface area of the sun is 11,916.8 times that of Earth (109.16 squared) and its volume is 1,300,893 times ours (109.16 cubed).

The Sun puts out 3.85 x 10^26 W
(385,000,000,000,000,000,000,000,000 Watts)
roll your mouse over the number to see all of it

At our distance from the Sun, we intercept 1370 W per square metre. Because of our clouds, snow, etc., we reflect 37% of that back to space. The 63% that is left is what gives us our temperature and powers our various systems (oceanic currents, atmospheric weather, plant life...).

The amount of energy flux (Watts per square metre) that a planet gets depends on the square of its distance from the sun. Twice as far, only one fourth of the flux.

Sirius put out 25.4 times more power than the Sun. If we could place a planet there with the same size and outer appearance (to reflect the same percentage of "Sirius-light"), then it would have to be placed a tiny bit over 5 times further from Sirius as we are from our Sun.

SQRT(25.4) = 5.04
5.04 times 150 million km = 756 million km (a little over 472 million miles).

That is almost the distance from our Sun to Jupiter (5.2 times 150 million km).

Sirius has a surface temperature of 9940 K (= 18,400 F) while our Sun has a surface temperature of "only" 5780 K (= 10,900 F).

The surface temperature determines the amount of energy flux that leaves the surface of the star (we have corrected for that by moving the planet 5 times further away). It also determines the distribution of that energy among the various wavelengths. Our Sun's peak energy is in what we call "visible light". Sirius's peak is what we call UltraViolet light.

So, even though we moved the planet 5 times further so that the "total" energy appears the same, the planet around Sirius will still get more UV, X-ray and gamma rays as Earth gets from the Sun.

Worse: Sirius has a companion star. It is faint (over 400 times fainter than our Sun) but it is hot 25,200 K (= almost 46,000 F). Whatever little light it emits is almost all harmful radiation.

Our planet's orbit (at 5.04 units from Sirius) would be inside the orbit of this second star (at 20 units from Sirius). For part of our year, this star would be in the sky at "night" so that we might be stuck indoors, fearing the radiation from Sirius during the day and from its companion at night.

At the distance we'd be from Sirius (5.04 units), given Sirius's mass (over twice that of our Sun), our year would be around 6 of our present Earth years.

Our orbit would be perturbed by the companion which, even though much smaller, still has the same mass as our Sun (the companion is a white dwarf -- small but very dense).

At its closest (15 units), we would be closer to it than Uranus is to our Sun. Maybe the perturbation would accumulate with every opposition (every 8 years or so) and our planet would end up being ejected from the system -- or thrown into the main central star, or simply put into such a complicated orbit that predicting seasons would become impossible.

Answer 2

The area around a star in which an earth-like habitable planet can form depends on how massive the star is and how old. a large star will have a habitable distance much further away than one that is the size of the sun. The planet itself must be large enough to have enough gravity to hold it's atmosphere.. if the air pressure is too low, then liquid water can't form which makes things pretty inhospitable even if you are at just the right distance from the sun.

Also, large gas giant planets near the habitable zone around a star can interfere with the formation of planets nearby. Multi-star systems like Sirius end up complicating the matter because planetary orbits that would fit within the habitable area may be unstable, and the habitable area may be oddly shaped leading to possibly habitable planets during part of the planets orbit and either too hot or cold the rest of the time.

Answer 3

Every star has a 'comfort zone', such that if a planet were to orbit in this area, liquid water would be present.

A star like Sirius, which is quite large, that zone would be far greater out from the star than we are from the sun. Likewise, a star like Barnard's star, a red dwarf, that zone would be far closer.

As far as the planet goes, it has to have a rocky make-up; and, it has to be of a particular size - it can't be so small that it can't retain an atmosphere, but a planet far larger than Earth may still contain life. Remember, the larger the planet's diameter, the farther you are from the center of gravity - since gravity increases with the square of the distance, so a large rocky world larger than Earth will only increase your weight by a fraction. (They found a 'super Earth' orbiting a star earlier this year, and although it was 5 times the size of Earth, our weight would only be about 1.5 times what it would be here.)

And, all life (as we know it), would require water.

Answer 4

i know that size matters know ever since they got rid of pluto but im not sure about the size however distance is not a requirement for a planet to be a planet. It depends on what there going around, it needs to circular around a star