Exoplanets: how the search for life became sexy

Hundreds of exoplanets have been discovered, but are we any closer to finding life? AAP

In the late 1980s, when I was a young whipper-snapper just starting out as an astronomer, it was quite obvious some fields had an incredibly high profile and others were outré.

The sexy ideas at the time included cosmology at high redshift and finding the next “record-breaking” distant quasar.

By far the most “out there” field to work in was the search for planets orbiting other stars.

Unloved and unwanted

The paradigm for a career in this field seemed to have been set by the Reformation-era philosopher, heretic and all-round nutcase Giordano Bruno.

Bruno is widely (though not very accurately) quoted as being the first person to suggest the existence of planets orbiting other stars, and was burned at the stake during the Roman Inquisition in 1600 (though for other “heretical crimes”).

While searching for planets as an astronomer in the 1980s didn’t mean you’d come to the same end as Bruno, it would pretty much guarantee you’d received strange glances at scientific conferences until the day you retired.

Bringing sexy back

A couple of decades later, things have changed. With hundreds of exoplanets discovered outside our solar system in the years since the first was announced in 1995, the field of “exoplanetary science” has found its groove.

Not only are astronomers now seriously expecting to answer questions such as, “How many habitable, Earth-like planets are there in the galaxy?”; they want to discover and observe potentially liveable planets.

How do we define “liveable”?

Before the discovery of exoplanets we had built up a quite complex model for the formation of our own solar system – one which reproduced the sizes, orbits and compositions of the eight planets we knew quite well.

Extending this model to other stars predicted (unsurprisingly) that most planetary systems would look like our own solar system. The discovery of the first hundred exoplanets, however, shattered this view.

Unlike our solar system, exoplanetary systems have:

  1. Gas-giant planets much closer to their stars than they should be able to form.

  2. Planets that mostly orbit in elongated elliptical orbits (as opposed to the mostly circular orbits of our solar system).

  3. Giant planets with densities ranging from that of fluffy Saturn (which is slightly less dense than liquid water), to compact Earth (as dense as silicate rock).

Unfortunately, none of this brings us any closer to learning how common a system such as ours is in the galaxy.

What are “habitable” environments?

In the absence of a deeper understanding we tend to fall back on a definition of habitability that mirrors the conditions on the surface of the Earth, by requiring that a “habitable” planet must have a rocky surface that can sustain liquid water.

It cannot orbit too close to its parent star (or else the surface will be too hot), nor can it be too distant from that star (or else the surface be too cold).

“Habitable”, by this definition, is just a proxy for “Earth-like”.

All this is why, when asked questions such as, “Is there life out there in the universe?” most astronomers – if they are honest – answer, “I just don’t know”.

Unfortunately, journalists and editors hate this answer, and usually say, “go on, guess”, and those guesses are what you usually see reported.

The Kepler Mission has taken great leaps forward in the search for Earth-like candidates. NASA Ames Research Center/Kepler Mission

Kepler

Investigating the prevalence of Earth-like planets in Earth-like orbits is the prime mission of the NASA’s Kepler Mission.

The Kepler satellite was launched in March 2009, on a three-year mission to stare continuously at a region of sky in the northern hemisphere in the constellations of Cygnus, Lyra and Draco.

The mission uses a 94-megapixel camera to take an image every six seconds, recording the light output for around 100,000 stars.

These measurements are of exquisite quality, and they need to be, because Kepler’s aim is to detect the tiny dimming of a star as a planet passes between us and the star.

This is why the recent announcement by the Kepler Mission of some 1,235 planet candidates from their first four months of data is of “candidates” only.

How do candidates graduate?

We now need to wait until those blips appear again to (a) confirm the first blip was real, and (b) estimate the planet’s orbital period.

However it pans out, Kepler’s search will in time result in an astounding statistical database on the prevalence of small, rocky and hopefully Earth-like planets.

At that point we’ll finally know the answer to at least part of the puzzle of whether our type of planet is common.

One thing’s for sure: the search for exoplanets is where it’s at.