Forty two planets: Not too hot, not too cold. They’re just right. They orbit distant suns — but not too distant to see. Is one of them Earth 2.0?

CHANCES are, it’s out there: Probably many, many more than one. Recent discoveries put the probable number of Earth-like planets in the Milky Way galaxy in the billions.

Why should ours be the only one to sustain life?

Somewhere in the vastness of space is another pale blue dot, glowing with water and an atmosphere rich in oxygen. It will probably be roughly the size of our own world and sitting somewhere within the narrow band which defines the distance from a star where liquid water can exist.

We’ve already found candidates. Sifted from among the pixels and bands of light captured from distant stars are the minute traces of planets potentially suited for life.

Over the past five years the Kepler Space Telescope, among others, has looked at some 150,000 stars. About 700 of these have planets — we’ve confirmed 1877 of them so far. There are traces of 4661 more.

Now NASA has just added another 500 to this list.

Eight of these planets show good potential of being “another Earth”, while 34 aren’t all that much behind.

But there are many more vying for attention.

Astronomers figure that roughly one in every five Sun-like stars in the galaxy have a ‘Goldilocks’ planet — one that’s not too hot, nor too cold. Just right.

That’s 11 billion globes.

EARTH 2.0: For the enhanced version of this story, click here

That’s not taking into account those around stars different to our own, or even moons.

Once you whittle away at the 300 billion stars in our home galaxy with all the criteria, exceptions and conditions necessary to sustain the delicate ingredients for life, astronomers believe 100 million habitable planets remain. And that’s a conservative estimate.

We’re only now beginning to try and “taste” the atmospheres of these alien worlds. Such efforts will produce the most monumental sign: The telltale traces of life.

One astronomer has gone so far as to put a date on it.

Dr Franck Marchis of the SETI Institute says we will find a planet similar to our own before 2024.

Australian astrophysicist Dr Alan Duffy, Research Fellow at the Swinburne University of Technology, agrees: “Absolutely. I would even argue sooner — I’m confident that we will get this confirmation of an Earth-like world with the new James Webb Space Telescope. Exactly how quickly we discover another Earth depends on how lucky we are with how many times we have to point JWST to come across another Earth. “

SETI’s Seth Shostak last year went even further out on a limb: Extraterrestrial intelligence will be located by 2040, he says. It may even be sooner now his organisation has been given a $100 million boost.

Whatever the predictions, the search for alien life is now NASA’s number one priority.

THE SEARCH

“Two decades ago ours was the only solar system known to astronomy that had planets. Today we have found thousands of alien worlds … that’s an incredible advance in our knowledge of how common our home, the Earth, actually is,” Dr Duffy said.

It was an event as big as Galileo first staring through a telescope to see the moons of Jupiter. We just don’t fully appreciate it yet.

In 1992, scientists found the first confirmed planets outside our own solar system. They were dead worlds, sterilised by the X-rays spewed out by the crushed remains of their star (a pulsar).

1995 clinched it: A planet was found orbiting the Sun-like star 51 Pegasi. But it also came with some big surprises. The planet was 150 times larger than Earth, but it was so close to its parent star that its orbit took a mere 4.2 days.

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That wasn’t supposed to be possible. Now, we’ve found many more like it.

In fact, most of the solar systems we’ve found look nothing like our own. Only recently have we found one with a “twin Jupiter” sitting protectively in a similar orbit.

Most of the exoplanets found so far are huge titans like Jupiter and Saturn. Many are far too close to their stars — zipping around in orbits that count years in terms of days and hours. They’re commonly called “Hot Jupiters”

But we’ve found plenty others. Some are prime candidates for life.

“Every star in the night sky is likely to have a planet,” Dr Duffy said. “Around every fifth star will be an Earth-sized world lying in a region hospitable for life.”

“This is the grail of modern astronomy,” SETI’s Dr Marchis said. “We’re trying desperately now to image those planets because we know they exist.”

AMONG THE CANDIDATES SO FAR

Kepler 452b: Announced July 23, 2015, this one tops the charts so far as it orbits a Sun-like star, has a similar year to our own (385 days) and sits squarely in the habitability zone. While some 60 per cent bigger than our own world, this is not expected to make much difference. All that needs to be determined now is the state of its atmosphere: Is it a runaway greenhouse? Or is it a blue dot with whisps of white?

EPIC 201367065d:This world belongs to a solar system holding three Super Earths. Planet “d”, some 40 per cent bigger than our own planet, is the best contender as it is sitting on the inside edge of the Goldilocks zone. Being just 147 light years away, the Hubble Space Telescope may be able to detect traces of its atmosphere.

Gliese 667: This one is an odd one. It is a nearby (23.6 light years) triple star system. The smallest star — a red dwarf — appears to have three potentially habitable worlds (GJ 667 Cc, Ce, Cf). Planet “Cc” has long been regarded as one of the most Earth-like yet found, even though it’s big: 3.8 times heavier than Earth.

Kepler 62: There are five worlds in this solar system, two in the habitable zone. Planet 62e is about 60 per cent bigger than Earth. Planet 62f may be just 40 per cent bigger. Both have the potential to be water worlds.

Kepler 186f:About 500 light years from Earth, this one ticks the right boxes. It is in a temperate, liquid-water supporting orbit. It’s also the closest in size to Earth found so far, being about 10 per cent bigger, and has a 130-day orbit around its parent red dwarf star. Some astronomers have declared it to be the best candidate for a habitable planet yet.

Kepler 296: This binary star system has two possibly habitable planets, 296e and 296f. The habitability of Planet “e” is suspect, with the odds of it having a hot Venus-like atmosphere being high. Planet “f” is somewhat more promising, sitting nicely within the Goldilocks band. But it is big: Possibly too big for life.

Kepler 438b: Discovered earlier this year, 438b is probably a rocky planet orbiting the inner-edge of its star’s habitability zone. Debate is raging, however, as to whether or not this planet — just 12 per cent bigger than Earth — is likely to be a desert world, or another Venus.

Kepler 442b:This one is exciting — one of the most potentially habitable exoplanets found so far. 442b is regarded to be a rocky planet only 30 per cent bigger than Earth. It also appears to be firmly ensconced within the narrow habitability zone of its dim K-type star.

HOW WE KNOW THEY’RE THERE

Doppler technique: When a planet orbits a star, its gravity also pulls on that star. While the planet is swung about in a big circle, the star makes a little wobble. This wobble affects the light it emits by a tiny — but detectible — fraction. Naturally, the bigger the planet and the closer it is, the bigger this wobble appears. But these worlds are not at all Earth-like.

Transit method: The Kepler telescope, launched in 2009, is capable of gazing at 150,000 stars at the same time. What it does is measure the amount of light being emitted by the star. When a planet crosses between the star and Kepler, it will dim ever so slightly. All it takes is time — and some very minute measurements — to track these planetary orbits and to determine a rough size and location. It also makes it easier to find smaller, Earth-sized planets.

Direct imaging: We have actually photographed worlds over interstellar distances. But, so far, these dozen or so blurry images are of supergiants orbiting at great distances from their stars — still glowing brightly from the inferno of their birth. Temperate Earth-sized planets orbiting close to their warmth-giving stars will be much more difficult — but not impossible. The telescopes capable viewing such tiny blue sparks have been designed — just not yet built. Such photos, once we get them, would contain the fingerprints of the planet’s atmosphere.

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While there are several methods of finding new worlds, it’s only the fingerprints within the spectra of light reflected from the atmospheres that will reveal the presence of life. And this is the next big challenge — both for funding and engineering.

“The information we get from the spectrograph is like listening to an orchestra performance; you hear all of the music together, but if you listen carefully, you can pick out a trumpet or a violin or a cello, and you know that those instruments are present,” exoplanet researcher Alexandra Lockwood wrote. “With the telescope, you see all of the light together, but the spectrograph allows you to pick out different pieces; like this wavelength of light means that there is sodium, or this one means that there’s water.”

But, in particular, they’ll be looking for something called an Oxygen “dimer” … two molecules of Oxygen bundled together and are believed to be only formed through the process of photosynthesis — plant life.

WHAT IS NEEDED FOR LIFE

Water is what we need to see. While this elixir of life has been found in the atmospheres of “hot Jupiter” planets orbiting distant stars, nothing has yet been found in the key “Goldilocks” zone.

But we’ve barely begun to look.

For liquid water to be present, a planet cannot be too close or too far from its star. Too close, the water boils away. Too far and it freezes (unless there is some pretty serious tectonic activity going on).

The very fact water has been found in exoplanet supergiants reveals it may be more common than anticipated. But isolating the light from Earth-sized planets from that of their stars has not yet been achieved. Once it has, then we’ll be able to begin the search for the fingerprints of water and life in earnest.

So it’s planets in the ‘Goldilocks Zone’ we’ll have to look for.

Finding them is not turning out to be a problem: We have 42 potential Earth 2.0 candidates so far.

But the full extent of their similarity to our home has not yet been established.

More data is needed yet to plug into the “Earth Similarity Index”, an equation designed to assess a planet’s suitability for life. It predicts (on a scale from zero to one with zero meaning no similarity and one being identical to Earth) how Earth-like a planet is based on its surface temperature, escape velocity, mean radius and bulk density.

Planets with an Earth Similar index from 0.8 — 1 are considered capable of hosting life similar to that found on our planet. In the equation Mars scores in the range of 0.6 — 0.8, showing it is too low to support life. Only seven — as yet unconfirmed worlds — fit this spectrum.

But the formula only calculates life like our own. Alien life may in fact be capable of living in a much broader range of conditions — such as an enormous world.

“Certainly life as we know it is still possible in a super-Terran world, as the surface gravity doesn’t change as much as you might imagine,” Dr Duffy said.” For example a super-Terran world may have ten times the mass of Earth and twice the diameter — so the gravity on its surface is only a few times that of ours. Any land-based life in this case may not grow as tall as on Earth, but would likely be stronger … But such a world can hold onto more of its atmosphere than Earth, which might create a runaway greenhouse effects and preventing life from forming.”

Here are some of the key influences:

Size and density: Big, heavy planets could contain life. But it gets tougher to live as gravity increases. And as a planet’s mass increases — so does the weight of its gravity and atmosphere. So some scientists have resolved to ignore any “Goldilocks” planet weighing in at more than twice our size.

Atmosphere: The ‘Earth 2.0’ candidates could have noxious greenhouse-gas atmospheres like Venus. They could be cold, dead worlds like Mars. We don’t know. Yet. What we do know is we need to find methane as potential evidence of life. We need to find water as evidence of potential habitability. Testing these atmospheres for such signs is the next phase of NASA’s 30-year exoplanet search plan.

Many astronomers say the search for Earth-like life is a mistake. It’s possible that Earth may not represent a typical habitable world.

A recent study has a new definition of the ideal planet: “Superhabitable”

Superhabitable would be a world two to three times bigger than Earth, somewhat older and orbiting stars smaller than our Sun. Water would ideally be scattered over the surface, not in deep oceans. What is vital, however, is the magnetic field to protect life from cosmic radiation. Inside their atmospheres, somewhat denser than our own, could be life — but not as we know it.

Even searching yellow “Sun-like” stars may be wrong. It seems red dwarfs have the greatest chance of generating planets in ideal orbits. Perhaps we’re the odd-ones out.

WHAT WOULD LIFE BE LIKE

In a word: Weird.

Whatever form it takes, life out there will likely be very different to that here.

Research from Caltech argues that the best place to find a habitable planet is around red dwarf stars. Life there would spawn under dim-red skies. Photosynthesis — if it exists — would be somewhat different. Plants could be black instead of green to catch as much light as possible.

Red dwarf stars are small.

So, for a world to be heated to Earth-like temperatures it would have to sit close to its star.

The catch?

When a planet gets so close it becomes ‘tidally locked’: One side would perpetually face the star — basking in its heat — while the other is fated to become a frozen waste.

Life would literally be on the edge, shifting in and out of the shadows to hunt and shelter. Intense winds would blast the heat from the sunny side to the night side, extending the “band of life” a little further.

But radiation will be the big unknown factor. Is the planet’s magnetic field strong or weak? Is the star active? This applies to all worlds, whether they orbit a red, blue or yellow star.

What about alien civilisations?

That’s a different ballgame.

Dr Duffy said: “To identify intelligent life is much harder, but if they have TV / radio signals our new radio telescopes such as the Square Kilometre Array will allow us to listen if we’re close enough.”

Those leading the hunt for extraterrestrial civilisations want to continue what we are have been doing for the past 40 years — just on a much larger scale with much more sensitive equipment and software: Point big radio dishes at the sky in the hope of picking up an “unnatural” signal.

But finding ideal planets first could help narrow the search.

WHAT IF WE FIND LIFE

It would be Earth-shattering.

Discovering life could change the world overnight. Our sense of self will shift. Our place in the cosmos will have been redefined.

We would not be alone.

“Personally, I can’t imagine how huge this impact on the world would be,” Dr Duffy said, “but I for one would certainly be relieved that the burden of ensuring the ongoing existence of life in a hostile universe is no longer humanity’s alone to bear.”

Finding alien intelligence would ramp things up even further.

Even though we would not be able to “talk” with aliens, a lot of debate is likely to be generated about everything from religion through to space-based defence systems.

Hysteria and paranoia would be a risk. But it could also inspire a new wave of space exploration and a desire to travel interstellar distances.

Chances are, according to some astronomers, we’ll personally experience whatever reaction it will be within the next 30 years.

But the questions are enormous.

“This is the biggest unknown of the famous Drake’s equation,” Dr Duffy said. “What does it mean to find life without intelligence?

“Is it simply that intelligence such as we possess is incredibly rare? Perhaps we can’t recognise other (alien) examples of intelligence? Or more terrifyingly, that soon after a civilisation gains the ability to communicate through radio signals, it also gains the ability to destroy itself? In the search for alien life we must ask these questions of ourselves as — so far as we know — ours is the only example of intelligent, communicating, life.”

THE FUTURE

Dr Duffy says the rate of discovery of interstellar worlds is exploding at a phenomenal pace.

“In two decades we have come so far, and yet what’s left is the hardest planet to find, a world like our world, Earth 2.0,” he said. ”To find such a small planet, far from the star (but not too far) we will need new technology and telescopes to aid us in our hunt.

“The hunt is on: Within a decade these telescopes are guaranteed to find Earth 2.0. Whether they find life is another issue.”

NASA’s recently released 30-year-plan has made one thing obvious: The search for life is now its primary goal. And it’s the same for the European Space Organisation (ESO) — as is evident from the projects almost ready to roll.

How we find 'Super Earth' The Transiting Exoplanet Survey Satellite (TESS) mission, scheduled for launch in 2017, is expected to 'discover thousands of exoplanets in orbit around the brightest stars in the sky'. Courtesy MIT/NASA

Planetary Transits and Oscillations (PLATO): The Kepler space telescope, hobbled by failures in key equipment, will soon have a successor. PLATO will scan the sky from 2024 in search of stars that have the telltale dips in brightness that happen when planets go across their parent star’s face. This will find more candidates for Earth 2.0, but will not be able to identify them itself.

James Webb Space Telescope (JWST): Slated for launch in 2018, this next-generation high-powered telescope should be able to directly observe much smaller exoplanets than we can now. The planets being identified now as potential candidates for life will be the first targeted when this telescope goes active.

Thirty Meter Telescope (TMT): This planned advanced reflecting telescope, while not yet fully approved, will be the biggest telescope on Earth. New technology will help filter out the influence of the atmosphere and give it the ability — hopefully — to peer at some exoplanets.

Transiting Exoplanet Survey Satellite (TESS):This sensor will be pointed at the brightest and closest stars over a two-year period from 2017. It will operate in a similar way to the current Kepler telescope — looking for the telltale variations in a star’s brightness to pinpoint the presence of planets. TESS is specifically designed to find the smaller planets in “Goldilocks” orbits.

Giant Magellan Telescope (GMT):This giant Earth-based telescope is currently under construction in Chile. It will combine seven separate advanced mirrors to produce pictures with 10 times the detail produced by the Hubble Space Telescope. The mirrors are designed to flex their focus to compensate for atmospheric turbulence above the telescope’s remote desert mountaintop. Astronomy Australia and The Australian National University are participants in this project.

@JamieSeidel