Nearby "earth-like" planet: not so much

Bad Astronomy
The entire universe in blog form
July 20 2011 7:06 AM

Nearby "earth-like" planet: not so much

There's some chatter on the web right now over a new scientific paper about a nearby exoplanet, and what I'm seeing are people speculating that it might be earth-like. Technology Review even titled their article "Astronomers Discover Habitable ExoEarth Orbiting Binary Star".

The problem with that is that the planet's not terribly earth-like, and it may not be habitable*.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


So what's the deal? I read the journal article (PDF), and this really is a good story, just not the one I'm seeing the chatter about.

55 Cancri is a nearby binary star at a distance of about 40 light years. One star is a dinky red dwarf, and the other is a fairly Sun-like star, though somewhat smaller and cooler. It's also much older, roughly 10 billion years old, more than twice the age of the Sun. It's actually at the point where it's starting to evolve into a red giant, and is called a sub-giant.

Back in 2007 it was announced that at least five planets orbit the bigger of the two stars (called 55 Cancri A; confusingly the red dwarf is 55 Cancri B (note the capital letter), while the planets are called b-f (lower case)). They range in mass from 0.026 to 3.84 times that of Jupiter (8.3 to 1200 times the mass of the Earth). 55 Cancri e is the lowest mass of these, but is extremely dense and hot, so not at all earth-like.

55 Cancri f is the interesting planet, though. The astronomers in question observed the star using an interferometer, allowing extremely precise measurements of the star's size, which in turn yielded very accurate numbers for its temperature and mass. All these together can be used to figure out its "habitable zone", the region around it where an orbiting planet would have liquid water on its surface.

Now right away, I'll say that finding the HZ (as we in the know call it) is not really straightforward. For example, a planet that has a thick atmosphere can be farther from its star and still have water due to the greenhouse effect; in fact, without air the average surface temperature of the Earth would be below freezing! And the greenhouse effect depends on what's in the atmosphere, its density, and so on. So I am wary of any declarations of planets being habitable based on this alone.

Still, let's see what we get. The authors published this diagram of the orbit of planet f:

The orbit of the planet is the solid black line, the grey area is the habitable zone of 55 Cancri A, and the dotted lines represent the orbits to scale of planets in our solar system (Mercury, Venus, Earth, and part of Mars) -- the red dwarf binary companion is way, way off the scale here. As you can see, due to its highly elliptical orbit, planet f spends about 3/4 of its time inside the star's HZ! That's pretty interesting. But what does it mean?

Specifically, with the new, more accurate numbers for the star, some assumptions about the planet's temperature can be made. For example, if it has no air at all and has no way to redistribute heat from the star, it has a temperature range of -10° to 86° C (18° to 187° F) as it swings around its star. That's quite a range!

As far as I can tell, it's unknown if this planet has an atmosphere or not. But if it can redistribute heat (as our atmosphere does for Earth), then those temperatures become -52° to 29° C (-60° to 84° F). These ranges of temperature, remember, are due to the changing distance of the planet to the star. If the planet is tilted, then you get seasonal changes too which can make these temperatures either more extreme or more moderate. But we can't know.

So clearly, the planet isn't exactly a garden spot. If it has air, the weather must be fierce!

But we're not done. What about the planet itself? Turns out it has over eight about 50 times the Earth's mass! and twice the radius (it's a hefty planet, actually roughly the same mass and size as its sister 55 Cancri e), giving it a surface gravity about twice that of Earth. Trying to lose weight? Stay away from 55 Cancri f! You'd weigh twice what you do now if you were there. [My apologies. Somehow I misread the planetary parameters for the planet e as the ones for f. Ironically, the reality makes my point even more strongly: at 50 Earth masses, 55 Cancri f is heftier than I thought, and so is not earth-like at all. It is far more likely to be a gas giant of some kind. It doesn't transit the star, so its actual size is unknown. My thanks to astronomer David Spiegel at Princeton for pointing out my error.]

Even though this planet may not remind us all that much of home, there are still mysteries to be pondered. How did two planets of nearly equal mass and size form in this system? Why is this one on such a highly elliptical orbit? Mind you, that planet is ten billion years old, twice as old as Earth! Did it get kicked into that orbit by some gravitational encounter with another planet? And how long has it been that way? Is this sort of configuration stable over billions of years?

And aside from all that, of course, the search for a truly earth-like planet will continue. My money is still on Kepler, but it'll be another year or so before we'll see anything. Patience can be a virtue; we've waited thousands of years to know if another Earth exists Out There, and we'll get our answer soon enough.

Image credit: NASA/JPL/Caltech

* And, to be nit-picky, it only orbits one of the stars in a binary pair.

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