Exoplanets, wobbles and spectrum analysis, OH my!

Are we alone in the universe? This question has probably been pondered since humans first realized that each of the bright twinkling dots in the nighttime sky was a distant sun, and has driven scientific discovery ever since.

While some wait for aliens to either pay us a visit or make an interstellar phone call, a group of scientists have given up waiting for ET to find us, and have started actively scouring the heavens for signs of extra solar system planets — also know as “exoplanets” — which might show the signs of life.


The concept that distant stars might have planets revolving around them is not as new a concept as you might think. Isaac Newton, in fact, advocated the theory in the early 18th century that each star was a sun in its own solar system, each with a system of planets all its own.

Despite a rash of 19th- and mid-20th-century astronomers claiming to have found exoplanets, the first confirmed planet outside of our solar system was found by Canadian astronomer Bruce Campbell — no, not the evil dead guy — in 1988. Using very precise measurements, Campbell noticed that some stars had a very pronounced “wobble,” while others had virtually none. Campbell and his team hypothesized that the wobbly stars were being pushed and pulled around by the mass of their orbiting planets, a theory that was confirmed a few years later.

Currently there are more than 400 discovered exoplanets, with new ones being found on an almost daily basis.

It also seems that, in terms of organization and makeup, our little solar system is pretty boring compared to the competition. Planets with many times the mass of Jupiter have been found, in addition to planets which are so close to their star that they orbit it every 20 hours — compare that to the 8,766 hours it takes our humble Earth to accomplish the same feat.

Perhaps the strangest planetary system discovered is HAT-P-7b, which has a planet that orbits its star the wrong way, or opposite to the star’s rotation. Imagine if Jupiter just decided to reverse directions one day . . .

The realization that planetary star systems are much stranger than we could have ever imagined has rejuvenated the field of astronomy, asking more questions about the formation of solar systems than they answer. And pointing out that our quaint neighbourhood is pretty boring and ordinary by the galaxy’s standards.

Detection methods

As proposed by Campbell in his 1988 paper, “Search for subsystem companions for solar-type stars,” finding planets via the wobble they induce on their suns has proved to be a very good way to determine if a star has planets in its orbit.

The most prolific planet-hunting observatory is the European Southern Observatory’s (ESO) high accuracy radial velocity planet searcher (HARPS), which discovered the first “super earth” — a term for a large rocky planet in an earthlike orbit around its star — in 2004. HARPS has since discovered 24 of the current 28 known exoplanets that have a mass below 20 Earths.
HARPS success can, in part, be attributed to its extreme sensitivity, being able to detect variations in a star’s movement as slow as 3.5 km per hour. However, looking for stellar wobbles, while it may be effective for finding large planets such as super-earths and gas giants, might not be the ideal method for discovering smaller, earth-sized planets in earthlike orbits. For those, we might need to actually go into space.

While it might not be the only kind of life out there, we are only familiar with the kind that grows on planets like ours. For this reason the holy grail of planet searching is an earthlike planet of similar size, and in a similar orbit. To discover these kinds of planets, very sensitive satellites, which look for a dimming of the star’s light as a planet passes in between the satellite and its star have recently been launched.

The French and European space agency’s convection rotation and planetary transits satellite (COROT) was launched in December 2006, and currently holds the prize for discovering the smallest exoplanet yet. At less than two times the mass of Earth, COROT-Exo-7b represents the potential for space-based planet hunting. Unfortunately, COROT-Exo-7b is the planet mentioned above which orbits so close to its star that it completes an orbit every 20 hours.

Therein lies one of the main problems with detecting planets as they move between their star and our satellites. To discover an earthlike planet in an earthlike orbit from our perspective, you get one chance a year to observe it move across its sun. Planets like COROT-Exo-7b are easy to detect, since several observations can be made in a matter of days, but the detection and conformation of a planet in an earthlike orbit could take several years.

To this end, NASA launched the Kepler satellite, whose multi-year mission is to detect alien earths.


Launched March 7, 2009, Kepler has been specially tuned to find Earth-like planets in orbits similar to that of our Earth. Kepler differentiates itself from the planet hunters which have gone before it by concentrating its efforts on a small group of stars in a single part of the sky. Also, Kepler is tuned to find small, cool and rocky planets, like our own, rather than the indiscriminate planet-hunting efforts which have gone before, which were simply looking for any kind of planet.

If all goes according to plan, Kepler will observe a group of 100,000 stars for three or more years, giving it an excellent chance of finding planets in that difficult to observe, but oh so important, one year orbital period. In fact, in a Scientific American interview, one of the project members, Alan Boss, stated that betting on Kepler finding an earthlike planet is almost a sure thing, and that in all likelihood several earthlike planets will be discovered. Boss bases his optimism on a prediction that puts the number of earthlike exoplanets observable by Kepler at about one in 1,000, meaning that if Kepler accomplishes its goal, and observes 100,000 systems, at least 100 earthlike exoplanets should be discovered by Kepler.

Boss believes that earthlike planets might be much more common than was previously assumed, citing the theory that earthlike planets should be much more common in the universe than gas giants, since they should be more likely to form in a new planetary system. He estimates that the number of earthlike planets out there in our own galaxy could number in the billions, and are just waiting to be found.

According to Boss, we won’t know how many earthlike planets Kepler has discovered, if any, until 2013, when it will be done collecting the multi-year orbital data.

What’s next?

Assuming Boss is correct, and a plethora of earths are out there waiting to be discovered, what can we do with that data? Since we can’t see them directly, only observing the effect they have on their star, asking Kepler or COROT if a planet is a Mars or an Earth, while an important distinction, is not possible.

Luckily we might be able to observe the light passing through the atmosphere of an exoplanet, allowing us to look for a shift in the light’s spectrum, which indicates the presence of molecules like oxygen and methane, sure signs of life.

A practical demonstration of this method was just applied to our own planet, in which the Spanish research centre instituto de astrofisica de canarias performed a spectral analysis of light passing through the earth’s atmosphere, sure enough the presence of methan and oxygen were readily apparent.

So with the discovery of earthlike planets harboring life potentially just a scant four years away, what does this mean for us? This could be our Galileo moment, where our understanding of the universe in which we live is fundamentally changed forever, and, while it might not be as Hollywood-perfect as the touchdown of a flying saucer, finding the evidence of life on another planet in our lifetime is still a pretty awesome achievement.