Tuesday, August 01, 2006

Possibility of habitible planets

This article on space.com caught my eye when it popped up in my news feed on my hompage. The article is titled "Habitable Planet Possible Around Nearby Star System". Reading over it, it's a pretty interesting article, but I figured I'd spend some of the time going into a bit more depth than the article does.

The first question I'm sure many people reading this article will have is "Is there a planet there?" The answer is "Possibly." However there's no direct evidence yet. Such small planets are far beyond our current threshold at the present time.

This may seem like a pretty lame answer. After all, why would astronomers start getting excited without any evidence to suggest there is something there?

The answer is that this is one of the rare instances when the response is something other than "absolutely not." So while it's not exactly positive confirmation, at least it's a step in the right direction.

So what determines whether or not there's any chance of a habitable planet?

Perhaps one of the first requirements is that it orbit the right kind of star. The really big, massive, hot stars give off most of their energy in the ultraviolet part of the spectrum. Life (as we know it) and UV don't really get along too well, so those really big massive stars aren't generally a place we care to look.

At the opposite extreme, there's the low mass red dwarfs. Intuition should tell you these are cooler, so any planet that's going to be a comfortable temperature will have to be huddled in close. Unfortunately for those plaents, lower mass stars like these are prone to immense solar flares and other activity that wouldn't help forming life out too much either.

So the stars can't be too hot, or too cold. There's a fairly wide range of comfortable medium however.

Thus, assuming we have the right kind of star, there will be a range of distances that the temperature and stellar conditions would be right for. However, a planet would have to actually be there to enjoy such a "habitable zone".

Unfortunately, this is what's ruling out most known solar systems from having habitable planets. There's something we're observing that telling us that any planets that did happen to form there, didn't stay.

The feature we're observing is massive planets, similar to, or larger than Jupiter, that are very close in to their parent star. And when I say close, I mean close. Many of the known ones are closer than Mercury to our sun.

So what's the problem with that? Why can't we have a Fatty McPlanet rubbin up against hot Mamma Sun while the little guys play out in the celestial yard?

The answer is that things just can't form that way. Models of solar system formation, based on what we understand from the forming solar systems we see in places like the Orion Nebula, show that these high mass Jupiter planets should form out where Jupiter is or further.

So what in the blazes are they doing way in there? The best explanation for this is that these planets made like the birds and migrated. Which begs the question, "How?"

Well, nothing says that these massive planets formed alone. Sometimes they can have brothers. And as we well know, brothers don't tend to get along to well. As we can understand from the discovery of Neptune, in which it was mathematically predicted by the graviational effects it had on Uranus, massive planets can influence one another due to gravity.

If they're close enough, and massive enough, their gravity can actually pull each other out of orbit. Ultimately, one will generally get thrown outward while the other heads inward, falling into a smaller orbit around the parent star.

So where do smaller habitable planets come into all this? The answer is that to get into those small tight orbits, the massive planet is going to have to cross the habitable zone. And if such a planet has enough mass to give a good toss to a similarly sized companion, just imagine what's going to happen to a pipsqueak planet the size of Earth. Such dimunitive planets can pick up enough velocity that they'd be ejected from the solar system all together! If they do manaage to stick around the solar system, chances are, they're not going to get lucky enough to stay in the habitable zone.

Thus, when we see these "hot Jupiters" its an indication that any planets that would be potentially habitable are long gone. Sadly, most solar systems we observe are like this.

But despair not my friend! Even though these are the ones we observe, there's a reason they crop up so often: They're the easiest to find! Ones with Jupiters out where our Jupiter is are much harder to detect. So even though we only see these ones, it doesn't mean they're the only ones out there.

And even if there's a massive planet rather close in, there's still hope. If it lies within the habitable zone, there's a chance that its moons may be able to support life.

So going back to the article, the system, 55 Cancri, is one of the few known ones that has a star that fits our Goldilocks criterion, and also has the Jupiter planets where we'd expect Jupiters to be to allow earthy planets (if they're there) to exist. Although it's not a definate confirmation, it's certainly better than the alternatives. This should give you a bit better appreciation for how lucky we are to be here (and no, that's not an argument from incredulity in favor of Creationism).

In next post regarding astronomical data, I'll probably be exploring how astronomers use the tools I've talked about to discover extra solar planets in the first place. So stay tuned!


Samira said...


Where is the "habitable zone" around our own sun? Do you define the habitable zone as the area in space around the sun that we could move the earth and it would still support (human) life?


Jon Voisey said...

For our own sun, it is about from the distance of Venus, out to about the asteroid belt. I generally define it as the distances in which a terrestrial sized planet with an atmosphere similar to our own would have a range of temperatures suitable for human life.

Samira said...

