New planet with Earthlike orbit? Nah.

Contributed by
Aug 3, 2007
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Some fuss is being made over a newly discovered planet around a red giant star (like here, for example, and on some online conversation I've had). Why?

It's not the star. This is the tenth such red giant star to have a planet found circling it.

It's not the planet. The planet has a minimum of 4.6 times Jupiter's mass (that's big), putting it firmly in the range of previous planets found.

Is it the orbit? Well, yeah. The orbital period is 360 days. And hey, the Earth's orbit is 365 days. So some folks, not surprisingly, assume that means this planet has an Earthlike orbit!

But that's only one way to characterize an orbit, and actually it's not a very useful one.

First, let's look at the star, called HD 17092. It is an old red giant, and has a mass 2.3 times that of the Sun. That means its gravity is stronger than the Sun's, of course. The speed at which a planet orbits its star depends on the star's gravity -- in fact, it increases as the square root of the star mass. If the star has four times the Sun's mass, than a planet at a given distance from that star will orbit it at twice the speed it would if it orbited the Sun (the square root of 4 = 2).

Since HD 17092 has 2.3 times the Sun's mass, a planet orbiting it at the same distance the Earth is from the Sun will move at a speed square root(2.3) = 1.5 times as quickly as the Earth does. If it were one AU (the Earth-Sun distance) from the star, it would orbit it in just 243 days. Since we know it actually orbits in 360 days, it must be farther from the star. If you do the math, you find that the planet is actually 1.3 AU from its star. So it's actually 30% farther out from its star than the Earth is from the Sun. That's kinda sorta Earthlike. Maybe.

You might think that would make the planet colder, but wait! We need to look at two more things.

The star is a red giant, which means it's cooler than the Sun. Its temperature is about 4600 Kelvin, compared to the Sun at 5500 Kelvin. Again, you'd think this would mean the planet would be cooler than the Earth. But (still) wait!

The star is a giant. That means it's bigger than the Sun, and in this case HD 17092 is about 10 times the diameter of the Sun. That means a planet orbiting it will receive more heat from it at a given distance. I have the equations written out if you care to look, but the bottom line is that the temperature of a planet compared to the Earth's depends on the temperature of the star and the square root of the star's size. When you do the math (and I hope I did it correctly), you get that the temperature of the planet is about 2.3 times that of the Earth (assuming lots of things like: it has the same reflectivity as Earth, it rotates rapidly, and other stuff that will affect this calculation, but probably not by much).

So actually, the planet's surface will be pretty hot: roughly 500 Celsius, or 900 Fahrenheit. That would melt lead and zinc! Remember, though, the planet has more than 4 times Jupiter's mass, or well over 1000 times Earth's mass, so it's probably a gas giant with no real surface at all.

Oh -- the orbit of the planet is more elliptical than Earth's orbit (eccentricity = 0.17 versus

0.09 0.02).

So. The planet is whoppingly huge; it's boiling hot; it has a star that appears 8 times bigger in its sky (the width of three fingers held at arm's length), but is much redder, than the Sun; it orbits 1.3 times farther out from its star than Earth does the Sun; and its orbit is more oval than Earth's.

But hey, at least the period is 360 days! That's Earthlike!


Mind you, I'm not making fun of people who were saying that (except for anyone who would write a headline saying it) because we're not used to thinking in terms of temperatures, distances, square roots, and stellar sizes. We hear the number 360 and that sounds familiar. But in every way that counts, the orbit isn't Earthlike at all.

This is an alien world, orbiting an alien star. There will come a time -- and it will be soon, I guarantee it -- when we will find an Earthlike planet orbiting a Sunlike star in an actual Earthlike orbit. And that, my friends, will truly be a day. And I'll be right here to talk about it.