Monday, April 23, 2007

Worlds Beyond

Every two or three weeks I take a quick look at the Extrasolar Planets Encyclopaedia. Most times there is nothing new at the site, but sometimes there is. Last week there were five new things there: five newly discovered worlds, orbiting stars 150 to 300 light years away.*

* Or 50 to 100 parsecs. Astronomers never use light years, only parsecs, meaning parallax of one arc-second - the distance at which Earth's orbit would look roughly the size of a dime as seen from a mile away. To sound like a pro, use parsecs - but light years are much cooler.

According to the Encyclopaedia, maintained by the Paris Observatory, 227 extrasolar planets are now known, plenty enough to make up for poor demoted Pluto. The California-Carnegie site, more conservatively, proclaims 202 "nearby" extrasolar planets - apart from the inevitable competition between research teams, there is always disagreement at the margins over both what is a planet and what counts as "nearby."

However you count them, this is two dozen times as many planets as there are in the Solar System, with or without Pluto. For such a profound discovery, the extrasolar planets don't get very much public attention, and it is easy to guess why: There are no dramatic pictures of them, in fact no pictures at all. All have been detected by more or less indirect means, and we have no idea what any of them look like (though there are some gorgeous if speculative images by space artist Lynette Cook).

For both astronomers and science fiction writers, the extrasolar planets have in fact been just a bit awkward. Most are giant planets, roughly as massive as Jupiter, but orbiting very much closer to their parent stars - half of them closer than Earth is to the Sun, and about a third of them closer than Mercury. No one expected this; when they started turning up the theorists had to scramble to explain them. For astronomers and SF writers alike it has meant that planetary systems are not the way we expected them to be.

In fairness, much of this embarrassment may be due to "selection effect." Our methods of finding them work best, and confirmation comes most quickly, for big planets close to a star. Thus those 200-odd planets are typical only of the kinds of planets that are easiest to find. If twins of Jupiter orbit nearby stars, they would be barely at the uncertain threshold of detectability - and even once initially detected it would take a decade or so to confirm that they are the real deal.

Yet here's a lurking uncertainty: Perhaps the Solar System is not typical, but something of a fluke, in which case Earthlike worlds may be few and far between. It's also a bit humbling that Ma Nature threw us such an unexpected curve.

All the same, it is awesome to think that there are over 200 known worlds out there, harbingers of countless more waiting to be found.


Carla said...

I think I'd favour the selective observation theory - these are the ones that are easiest to find. Would an Earth-like planet even be detectable?

Rick said...

Only if several lucky breaks were combined. The method that has led to the most discoveries is measuring the planet's gravitational tug on its parent star, which favors big, close planets. So does another method, detecting the slight dimming of the star's light as the planet passes directly in front of it.

The subtlest of the selective factors is time. The teams may (and have at times hinted that they are) tracking "normal Jupiters" - giant planets orbiting about 5 times Earth distance from their parent stars, in nearly circular orbits. Planets like that could indicate orderly systems like ours; most of the currently known ones have orbits suggesting a chaotic early history.

But it takes about a full orbit to confirm a detection, 12 years for a Jupiter twin, and the high-precision searches haven't been going much longer than that.

Amazingly, though, the Compuserve front page reports discovery of a "potentially habitable" planet - about 5x Earth mass, but orbiting in its star's habitable zone. It's not up on the Extrasolar Planets Encyclopaedia site yet, though, nor at Sky & Telescope - I won't make a front-page post about it till it is better sourced.

But this is way cool!

Canageek said...

I don't know if you've been following this, but there have been some *amazing* discoveries of terrestrial planets since you posted this.

Also; I looked at the date and realized that I was in 1st year uni when you posted this, so I hope you don't mind me commenting as I go. I found your site back when I was in high school, but only revisted it recently and discovered your blog.

Unknown said...

Of course, if our solar system is unusual, it could help explain the Fermi Paradox.

Canageek introduced me to your site, so I'll be commenting on some old posts as I go.

Ada said...

Bah. Blogger doesn't link my name to my Google account. I'll be using Name/URL from now on.

Rick said...

A rather belated welcome to the comment threads!

Your screen name links to a website called Phoenix Farm, which I infer to be about ... a farm.

Canageek said...

Rick: Ada lives and works on a family farm, thus the link to it.

Rick said...

A perfectly sensible reason! I only mentioned the link because Ada noted having linking problems.

But come to think of it, a farming background could be relevant to the habitat discussions that sometimes come up around here.