24 July, 2015

Is Kepler-452b really Earth 2.0?

With all the hype in the media today around finding “Earth's Twin”, lets have a deeper look at what Kepler has discovered.
 
From the ScienceDaily article...

Bigger, older cousin to Earth discovered

NASA's Kepler mission has confirmed the first near-Earth-size planet in the "habitable zone" around a sun-like star. This discovery and the introduction of 11 other new small habitable zone candidate planets mark another milestone in the journey to finding another "Earth." 
 
The newly discovered Kepler-452b is the smallest planet to date discovered orbiting in the habitable zone -- the area around a star where liquid water could pool on the surface of an orbiting planet -- of a G2-type star, like our sun. The confirmation of Kepler-452b brings the total number of confirmed planets to 1,030.
 
"On the 20th anniversary year of the discovery that proved other suns host planets, the Kepler exoplanet explorer has discovered a planet and star which most closely resemble the Earth and our Sun," said John Grunsfeld, associate administrator of NASA's Science Mission Directorate at the agency's headquarters in Washington. "This exciting result brings us one step closer to finding an Earth 2.0."
 
Kepler-452b is 60 percent larger in diameter than Earth and is considered a super-Earth-size planet. While its mass and composition are not yet determined, previous research suggests that planets the size of Kepler-452b have a good chance of being rocky.

While Kepler-452b is larger than Earth, its 385-day orbit is only 5 percent longer. The planet is 5 percent farther from its parent star Kepler-452 than Earth is from the Sun. Kepler-452 is 6 billion years old, 1.5 billion years older than our sun, has the same temperature, and is 20 percent brighter and has a diameter 10 percent larger.  

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A few thoughts come to mind about the discovery of Kepler-452b.
 
Firstly, it is a fantastic discovery and great to see we are close to detecting Earth sized planets. The majority of planets that have been detect are "Super Earths", Neptune sized or Jupiter sized. Roughly eight or so planets slightly smaller than Earth have also been discovered according to the Exoplanet Catalogue, but these are all either too close to their parent star, or too far away to be suitable to support life.
 
Finding a planet close the mass of Earth, in the zone where water (if present) can exist in liquid form, is a great discovery and full applause to NASA and the Kepler team.
 
So the big question is could Kepler-452b support life?

First up, scientists will need to determine if Kepler-452b is a rocky planet like Earth, Venus, Mars and Mercury. If it is not, then no it can't support life.
 
If Kepler-452b is a rocky planet, then in Kepler-452b's favour that it could support life are the facts that it appears to have a fairly stable, circular orbit and that Kepler-452 (its parent star) is a Type G2 star which is a similar spectral class to our own Sun.
 
Not in Kepler-452b's favour is that it appears so far to be the only planet in the Kepler-452 system, its size is too big, and Kepler-452 is now likely more active that our Sun. These things don't bode well for advanced life. And also pose a real challenge to simple life, such as bacteria.

If Kepler-452b turns out to be the only planet in the Kepler-452 system, then it will not be protected like Earth is from regular asteroid or comet impact. Jupiter and the other gas giants in our system act like shields their mass, acting via gravity, to either deflect or absorb the vast majority of stellar debris (asteroids and comets) that would otherwise come in our direction.

Its uncertain at this time what mass Kepler-452b has. All we know so far is that its about 60% larger than our Earth. Such a large size will likely mean that Kepler-452b, if its a rocky planet, will have a greater mass than Earth. If so, then it would capture and retained a much thicker atmosphere than Earth and a much larger water content. Too much of both is not good for life, even simple life. Just a cursory look at Venus will show you that a thick atmosphere is not a good thing for life.

Originally when Earth formed it had an atmosphere of about 100 times thicker than our current atmosphere. By comparison, Earths original atmosphere would have been about 2 to 3 times thicker than Venus. Moreover, Earth originally would have had a water content of somewhere between 5% to 15% of the mass of the planet. The water content of the Earth by mass is now far less. As Universe Today notes '..while the oceans cover 71% of the Earth’s surface, they only account for 0.02% of our planet’s total mass. ' The reason Earth has a much thinner atmosphere and lower water content today is due to how the Moon was formed. And these are very important characteristics for life on Earth.

Kepler-452 is now in its older years. As Type G stars last for about 10 billion years, if Kepler-452 was a human, it would be in its sixties. Type G stars at this stage will be flaring more actively and that's not a good thing for life, especially advanced life. Its detrimental to an atmosphere especially if Kepler-452b has no or little magnetic field like Mars. UV radiation is also a killer to life. If there is no adequate ozone layer, the survivability of any life is not good. If we can get a spectral image of Kepler-452b scientist should be able to work out the primary chemical composition of its atmosphere, assuming it has one. If little or no oxygen is present in the atmosphere, then not only will this show there is no life on Kepler-452b, but also no ozone to protect life.

Another thing to consider about Kepler-452b is to ask if it has plate tectonics. Plate tectonics is essential for life, forming landmasses above the oceans, and burying excess carbon. For plate tectonics, you need lots of thorium and uranium to provide the heat that drives the mantle of the planet. You also need a strong magnetic field to i) prevent the atmosphere from being sputtered away in to space, and ii) protect the planet from solar and cosmic rays. Earth has a rich iron core that produces a long lived magnetic dynamo that protects life on our planet. Mar's magnetic field is almost non-existent. The reason Earth has such a core is due to the Moon formation event. Whether Kepler-452b has such a core will be an open question we may never be able to answer, but is one that is crucial to life.

Your also going to need a moon capable of stabilising the axial tilt of Kepler-452b and slow down it rotation rate enough in order to prevent if from having raging cyclonic storms always raging across most of the surface of the planet. Mars, because its two moons are too small, flip flops on its axis by 40 to 60 degrees off the solar plane. Earth by comparison oscillates by only a degree around its axial tilt of 23 degrees. This tilt and oscillation provides Earth with little variation in its climate extremes, put still permits seasonal variability between the hemispheres. A flip flopping axial tilt really complicates matters for life, and again, that's not a good thing.

I could go on with more examples but this post has gone on much longer than I wanted. So for the sake of brevity, I'll point you in the direct of a few books you can read at you leisure. And as always, the Reasons to Believe website has a plethora of articles about Exoplanets and the fine tuning of our Solar System and Earth that make it habitable.

So, for the above and other reasons, I doubt that Kepler-452b will be suitable for life, even simple life. More research into Kepler-452b over the coming years will help answer some of these points, either in the for or against camp.

The discovery of Kepler-452b is science at its best. And lets be excited about that. But in my opinion, its rather overreaching to call Kepler-452b Earth 2.0. The quest for an Earth twin continues, but I don't believe that we will ever find one.

Book resources:

Why the Universe is the Way it is

How to build a Habitable Planet

What if the Moon didn't exist?

Rare Earth: Why Complex Life is Uncommon in the Universe
 

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