Science > Earth: Planet or a Star?

Earth: Planet or a Star?

Published: January 11, 2009

Is Earth a planet or a star? It may seem like a silly question but first you have to consider the difference between the definitions of planets and stars. If you look closely at the classifications its actually very hard to draw definitive lines.

The common preconceived notion is that a star is massively dense and radiates light due to nuclear fusion. Of course this is the case for a proportion of stars but hardly flat-out rule for all stars. Interestingly, Wikipedia currently has the one-liner for the definition of a star to be; "A star is a massive, luminous ball of plasma that is held together by its own gravity."

Looks like the preconceived notion isn't that far off.

The case has to made that not all stars shine. At least not as light visible to our own eyes. Visible light is just a portion of the electromagnetic spectrum. We can say, to be a star, some form of electromagnetic radiation must be emitted by the stellar object and it can't just be the reflection from another object.

This takes care of the idea of luminosity but what about mass? Is mass essential to the classification of a star?

Weighing up the idea of a star as based on its mass seems to have little merit. What reason could there be to exclude a luminous stellar object just because it mass is too small? Is it simply important the object is massive enough to hold itself together while emitting radiation? This seems most likely and is also backed up by the Wikipedia definition of a star, above.

Interestingly its very hard to find the line between the definition of stars and planets as mentioned on the Brown Dwarf Wikipedia page:

Some astronomers believe that there is in fact no actual black-and-white line separating light brown dwarfs from heavy planets, and that rather there is a continuum. For example, Jupiter and Saturn are both made out of primarily hydrogen and helium, like the Sun. Saturn is nearly as large as Jupiter, despite having only 30% the mass. Three of the giants in our solar system (Jupiter, Saturn, and Neptune) emit more heat than they receive from the Sun. And all four giant planets have their own "planetary systems" -- their moons. In addition, it has been found that both planets and brown dwarfs can have eccentric orbits.

Currently, the International Astronomical Union considers objects with masses above the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) to be a brown dwarf, whereas those objects under that mass (and orbiting stars or stellar remnants) are considered planets.The International Astronomical Union may laugh at the idea that the Earth is anything but a planet. If there is uncertainty about the boundary between stars and planets does it make sense in any context to think of the Earth itself as a star? We emit substantial amounts of radiation in a fairly wide spectrum (via such things as city street lighting through to terrestrial communications). Not only do we emit radiation, we produce it from the resources within the planet - it is not the reflection of radiation from our Sun (excluding the borderline case of solar power). Technically we have the power of the stars. Why then, should we not be classified as one?

Comments are welcome!


1. Kay on January 15, 2009

Interesting proposal Luke, I think it needs more work, but you bring up valid points. I'm not sure we could consider emitting man-made radiation the same as the earth itself producing it, but there is more to the argument than meets the eye.

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