The search for a second Earth

Credit: Oliver Burston

Are there Earth-like planets throughout the Universe? We may soon find out. 


Exoplanets

The year 2009 could be very special indeed. The exciting consensus among exoplanetary experts is that we could find another Earth-like planet capable of bearing life before the year is out. The race between rival research teams is on, and the competition to be first to discover an alien Earth is fierce. It’s even possible that by 2015, scientists could have convincing evidence that an Earth-like world bears extraterrestrial life.

This story really begins in 1992, when the first world orbiting another star beyond our Solar System was discovered. Since then, astronomers have been discovering so-called exoplanets at an ever-quickening rate. Today, 329 exoplanets are known, but none are much like Earth. Most are gas giants, larger even than Jupiter. The smallest is still over three times more massive than Earth, and so it’s been dubbed a ‘super-Earth’.

“Many different competing groups are rushing to claim the prize of discovering the first Earth-mass exoplanet, but the really exciting discovery would be a true Earth analogue,” says Don Pollacco, team leader of the SuperWASP exoplanet survey at Queen’s University, Belfast.

A ‘second Earth’ would be a rocky planet with a similar mass to Earth. It would have to orbit a Sun-like star at the right distance, so it receives just the right amount of heat to support liquid water on its surface – water being a prerequisite for all life as we know it. The range of suitable orbits around a star is known as the habitable zone. For a Sun-like star, any terrestrial planets orbiting at the right distance for a habitable climate would consequently have an orbital period identical to ours: ie, one year.


How to find exoplanets

Radial Velocity

A planet in orbit gravitationally tugs its star periodically towards, and then away from, the line-of-sight from Earth. It can be detected by the Doppler shift in the starlight.

 

Transit

If a system of planets is aligned just right, along the line-of-sight from Earth, an orbiting planet will pass across the disc of its star, dimming its light fractionally as it does so.

Microlensing

As predicted by Einstein’s theory of relativity, the gravity of a star and its planets focuses the light from a background star as they pass in front of it. Sadly, tracking them as they move on would be nigh-on impossible.

 

Astrometry

Planets exert a minute pull on their parent stars, which should be detectable if the system is face-on to us. Technologically this is extremely tricky, and this method has yet to make a single detection.

 

 


The Gliese-ian candidate

The most Earth-like planet found so far has the rather uninspiring name Gliese 581c. This is a super-Earth, at least five times more massive than Earth, and by some calculations may have a climate suitable for oceans and life. However, no-one knows its real size with any certainty because it was discovered by a technique called radial velocity (see ‘How to find exoplanets’, below), which can only ever tell us its minimum mass. So Gliese 581c may not even be a rocky, terrestrial planet.

“If it’s much heavier than five Earths it may have gathered a thick atmosphere of hydrogen during its formation and so it would more closely resemble Neptune,” explains Didier Queloz, who was part of the Geneva Observatory team that discovered it. >

Although it’s potentially within the habitable zone, Gliese 581c orbits a red dwarf – a star much smaller and cooler than our Sun – and its orbit only takes a fortnight. Nonetheless, Queloz believes further Earth-like worlds will be discovered soon. “If we’re talking about roughly Earth-mass planets orbiting in the habitable zone of stars a little smaller than the Sun, I’m confident we’ll find one using radial velocity within two to five years,” he says.

Much more can be determined about an exoplanet if it also happens to transit in front of its star, as seen from Earth’s line-of-sight. By measuring the periodic dimming of the starlight, researchers can work out the true size of the planet. Knowing both the mass (from the radial velocity) and the diameter (from the transit measurements) of the planet enables astronomers to calculate its density, and therefore whether or not it’s rocky, like Earth. Ground-based scopes searching for planetary transits, such as Pollacco’s SuperWASP, can detect a minimum starlight dimming of about one per cent. Due to the relative sizes of planets and their stars, this means that current transit searches of Sun-like stars can only spot planets down to the size of Jupiter. But for smaller types of star, such as M-class red dwarfs, a one per cent signal dip does correspond to an Earth-mass planet.

As well as terrestrial telescopes, the space-based COROT is also capable of bagging Earth-mass planets in close orbits (see ‘The planet-hunters’, below). “For this reason, I believe the first terrestrial-sized exoplanets could be discovered around M-dwarfs this year, but certainly within the next 15 years,” states Pollacco. But the problem with cool red dwarfs is that to lie within their habitable zones, planets must orbit very tightly, posing problems for habitability. What’s needed is a telescope capable of detecting Earth-mass planets orbiting Sun-like stars.


Planet Hunters

The telescopes that will be searching for exoplanets – and perhaps extraterrestrial life – over the course of the next decade

KEPLER

A NASA space telescope scheduled for launch in March 2009, Kepler will use its metre-wide telescope to hunt for exoplanets using the transit technique. If such planets are fairly common, then by 2012 Kepler is expected by some experts to discover as many as 50 true analogues of our world: Earth-like planets orbiting in the habitable zone of Sun-like stars.

 

 

COROT

The COROT (‘COnvection, ROtation and planetary Transits’) mission is a French-led space telescope launched in December 2006. COROT is using its 30cm telescope to stare at a patch of stars for 150 days at a time in order (hopefully) to spot lots of new transiting exoplanets. It will only be able to detect super-Earths orbiting close to their stars.

 

 

JWST (James Webb Space Telescope)
Designed as the successor to Hubble, the JWST will be launched in 2013 with a large 6.5m mirror and a sunshade the size of a tennis court. JWST will not search for new exoplanets, but its infra-red capability will be critical in characterising the atmospheres of planets discovered by other instruments.

 

 

DARWIN & TPF

NASA’s Terrestrial Planet Finder and ESA’s Darwin (shown below) are proposed space telescopes of a very similar design. Using a flotilla of independent telescopes, they could null the starlight to photograph Earth-like planets, analysing their atmospheres’ chemical composition to search for biosignatures. Neither is likely to launch before 2015.

 

 

 

 


Staring into space

The Kepler space mission, due for launch next month, has been designed with this goal in mind. Kepler will stare at the same patch of 100,000 stars for the whole three and a half years of its mission. This means that not only will it be able to survey vast numbers of Sun-like stars at once, but it can detect transiting Earths with orbits of one year. Dimitar Sasselov, professor of astronomy at Harvard University, is optimistic about there being a major discovery very soon. “I think the first Earth-mass planet orbiting in the habitable zone of a solar-type star will be found by Kepler in 2013,” he says.

Sasselov’s team will follow up with radial velocity measurements to confirm planets’ masses. Over the complete course of the mission, Kepler scientists are expecting to find up to 50 true Earth-analogue systems.

It is possible, though, that Kepler will be pipped to the post for the ultimate planet-hunting prize. There’s another technique that’s already sensitive to the tiny influence of Earth-sized planets in the habitable zone of their stars. Gravitational microlensing (see ‘How to find exoplanets’) relies on an aspect of Einstein’s General Relativity: massive objects bend light. If a previously unseen star passes between your telescope and a distant star, its gravity briefly focuses and brightens the starlight being observed. Any accompanying planets will cause tiny bumps on this light curve, even if they are only Earth-sized. These transient stellar alignments occur randomly.

This means that for any chance of spotting a microlensing event, astronomers have to stare at regions of the sky rich in background stars, such as towards the central bulge of our Galaxy. This strategy means that any planets that are discovered are very distant – up to halfway across the galaxy. And after the microlensing event, the passing star slips back into obscurity, making any follow-up studies utterly impossible. There is every chance that during 2009 astronomers will discover the very first truly Earth-like planetary system but that we may never see it again.

Nonetheless, gravitational microlensing remains an incredibly powerful search tool as it has the potential to generate great databases of alien planetary systems, building up statistics on just how common Earth-like planets are in the Galaxy.


The Expert:

Dr Giovanna Tinetti is a senior research fellow in exoplanets at University College London

When do you think a ‘second Earth’ will be discovered?

It really depends on what you mean by ‘Earth-like’. An Earth-sized exoplanet is already within the detection limits of current projects using the transit technique and looking at red dwarf stars, or by using gravitational microlensing. It’s possible a research team may announce the discovery of the first Earth-mass planet during 2009. The tell-tale signs of such a planet might already be lurking in the data on a researcher’s hard disk, just waiting to be analysed!

But you’re more interested in a true Earth-analogue?

Yes, although many research teams are racing to be the first to announce an Earth-mass detection, the size of the planet is not the be-all and end-all. Much more exciting would be the discovery of a really Earth-like planet: orbiting a Sun-like star at the right distance for liquid water, and thus life, to be possible on its surface. That could happen this year, or it might take five or more years. I think it’s also important not to get too fixated on terrestrial-mass exoplanets: 3-4x Earth-mass planets may also be perfectly suitable for life, we just don’t know yet.

Once another truly Earth-like world has been detected, what comes next?

After detection comes characterisation – we want to get to know what this new planet is like. Although we may find the first exo-Earth during 2009, it may be at least another decade before we can properly investigate it in detail. We would like to be able to analyse the spectrum of light reflected by the planet as this will allow us to determine the gases making up its atmosphere. Detecting both oxygen and methane in the air of an exoplanet is thought by many researchers to be a strong signature of the action of life. This might be possible using upcoming instruments like the James Webb Space Telescope or the ground-based Extremely Large Telescope, or several specialised exoplanetary space missions that are still undergoing design and funding reviews.


Visiting a home from home

Let’s say an Earth-like exoplanet is discovered just 10 or 20 lightyears away, and is subsequently found to possess an atmosphere rich in water, methane and oxygen indicative of life. What’s the next step? It’s not entirely unfeasible that we could build a starship to sail to this neighbouring Eden using technology not far beyond our current grasp, without requiring the kind of physics, such as warp drives or wormholes, that currently resides in science fiction.

Our first interstellar mission would be an automated explorer, built to be exceedingly hardy and reliable for the long voyage. The robotic starship would have to be well shielded against the energetic cosmic radiation and dust and gas particles of interstellar space, especially at the high speeds required to make the journey in anything less than centuries. The spacecraft would need an artificially-intelligent computer to control the mission as the communication times with Earth get longer and longer, and the capability for self-repair to keep it running for hundreds of years. We would also need to pack a long-lived nuclear power source to last the journey.

Our exoplanetary explorer could be flung in the right direction thanks to a close encounter with the powerful gravity of Jupiter and accelerated up to a substantial fraction of the speed of light using something like a pulsed nuclear drive, or light sails pushed by lasers. The probe would obviously need to slow down again as it approached the target system, and ideally even insert itself into orbit around the terrestrial planet.

From this close vantage point, the probe could observe the planet for years. It could scrutinise the dynamic chemistry of the atmosphere and oceans, map the continents and seas, track seasonal variations in vegetation coverage, and possibly send a few landers down to the surface for a closer look at the most interesting regions. The mother ship would intelligently analyse all this raw data, and beam the most interesting information home to scientists waiting back on Earth. And who knows, if this second Earth were found to be able to support human life we might one day send inhabited ships to colonise the ‘New World’.


The next step

After discovering a second Earth – what then? The answer is, of course, to find out what it’s like. To rank as a true second Earth, an exoplanet would not only need to be the right size, and in the right kind of orbit around a Sun-like star, but also have an atmosphere that is very similar to ours.

Giovanna Tinetti, now at University College London, led a team of astronomers who, in 2007, were the first to discover water in the atmosphere of an exoplanet. Last year, the organic molecule methane was also detected on the same world.

Tinetti explains how the technique works. “Many gas molecules absorb infrared light of a particular wavelength. By looking at the infrared spectrum of light coming from an alien world, we can read the chemistry of its atmosphere,” she explains.

Tinetti may have found water and methane in the atmosphere of a planet, but it was a hot gas giant, so there’s no hope of finding life there. Once an Earth-like rocky world is discovered, however, certain features of its atmosphere could betray the existence of alien life. Many organisms on Earth release methane, while others produce oxygen as they photosynthesise with sunlight. “These two gases react rapidly with each other, and so if we spot the fingerprints of both these gases, then something must be producing them in great volumes – probably biology,” explains Tinetti.

With this possibility, however, may come the most frustrating part of the search. If the planet is discovered using microlensing there is no hope of follow-up observations. Likewise, if the existence of the planet is inferred by the radial velocity technique, but it doesn’t also transit its star, there is little we could do to characterise it. The only way we could then isolate a planet’s feeble light from the glare of its star would be to directly image it. That would require new instruments, but the James Webb Space Telescope won’t be launched until 2013, and giant ground-based telescopes are even further away.

Laments Tinetti, “Imagine the intense frustration of knowing there’s another Earth-like world out there, but having to wait for over a decade before we can start reading its atmosphere for signs of life.”


This article first appeared in the February 2009 issue of Sky at Night magazine

Photo Credits: detection methods:Paul Wootoon, Nasa x 2, CNES 2006/Illustration D. Ducros, ESA 2002/Illustration by Medialab, NASA/ESA & K.Sahu (STSCL)
 
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