Why It Is Misleading To Compare Exoplanet Kepler-452b To Earth

NASA’s announcement of the discovery of a new extrasolar planet has been met with a lot of excitement. But the truth is that it is impossible to judge whether it is similar to Earth with the few parameters we have – it might just as well resemble Venus, or something entirely different.

The planet, Kepler-452b, was detected by the Kepler telescope, which looks for small dips in a star’s brightness as planets pass across its surface. It is a method that measures the planet’s size, but not its mass. Conditions on Kepler-452b are therefore entirely estimated from just two data points: the planet’s size and the radiation it receives from its star.

The habitable-zone myth

Kepler-452b was found to be 60% larger than the Earth. It orbits a sun-like star once every 384.84 days. As a result, the planet receives a similar amount of radiation as we do from the sun; just 10% higher. This puts the Kepler-452b in the so-called “habitable zone”; a term that sounds excitingly promising for life, but is actually misleading.

Looks familiar? Artist’s impression of the new exo-planet and its star NASA

The habitable zone is the region around a star where liquid water could exist on a suitable planet’s surface. The key word is “suitable”. A gas-planet like Neptune in the habitable zone would clearly not host oceans since it has no surface. The habitable zone is best considered as a way of narrowing down candidates for investigation in future missions.

Kepler-452b’s radius puts it on the brink of the divide between a rocky planet and a small Neptune. In the research paper that announced the discovery, the authors put the probability of the planet having a rocky surface about 50%-60%, so it is by no means sure.

Unpredictable geology

Rocky planets like the Earth are made from iron, silicon, magnesium and carbon. While these ingredients are expected to be similar in other planetary systems, their relative quantities may be quite different. Variations would produce alternative planet interiors with a completely different geology.

For example, a planet made mostly out of carbon could have mantles made of diamond, meaning they would not move easily. This would bring plate tectonics to a screeching halt. Similarly, magnesium-rich planets may have thick crusts that are resilient to fractures. Both results would limit volcano activity that is thought to be essential for sustaining a long lasting atmosphere.

Venus and Earth. Similar but oh so different. wikimedia

If Kepler-452b nevertheless has a similar composition to Earth, we run into another problem: gravity. Based on an Earth-like density, Kepler-452b would be five times more massive than our planet.

This would correspond to a stronger gravitational pull, capable of drawing in a thick atmosphere to create a potential runaway greenhouse effect, which means that the planet’s temperature continues to climb. This could be especially problematic as the increasing energy from its ageing sun is likely to be heating up the surface. Any water present on the planet’s surface would then boil away, leaving a super-Venus, rather than a super-Earth.

No neighbours

Another problem is that Kepler-452b is alone. As far as we know, there are no other planets in the same system. This is an issue because it was most likely our giant gas planets that helped direct water to Earth.

At our position from the sun, the dust grains that came together to form the Earth were too warm to contain ice. Instead, they produced a dry planet that later had its water most likely delivered by icy meteorites. These frozen seas formed in the colder outer solar system and were kicked towards Earth by Jupiter’s huge gravitational tug. No Jupiter analogue for Kepler-452b might mean no water and therefore, no recognisable life.

All these possibilities mean that even a planet exactly the same size as Earth, orbiting a star identical to our sun on an orbit that takes exactly one year might still be an utterly alien world. Conditions on a planet’s surface are dictated by a myriad of factors – including atmosphere, magnetic fields and planet interactions, which we currently have no way of measuring.

That does not mean that Kepler-452b is not a fantastic find. It has the longest year of any transiting planet of its size, holding the door open to finding more diverse planetary system. However, whether these discoveries are truly like Earth is a problem we cannot yet tackle.

The ConversationElizabeth Tasker is Assistant Professor at Hokkaido University.This article was originally published on The Conversation. Read the original article.

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Comments

    Thank you!
    Finally someone has written something about the new planet that isn't sprinkled with fairy dust. All the 'Earth 2.0' garbage that has been turning up on line and on telly is just pure speculation. ABC TV even had a 'fly over' animation with water and land masses without (AFAICT) not a hint of 'artist's impression' disclaimer.

      Coupled with the fact that it's one THOUSAND four HUNDRED light years away. Given our fastest spacecraft ever made travelled at about 70km per second and the speed of light is about 300 THOUSAND km per second kinda puts this place out of reach...
      At current speeds (which were only reached firing things towards the centre of the solar system and not out) it would take about 6 million years to reach.
      But it's good to dream.

        If governments put more money into science and less into military world domination then we may have had a chance this century. Not at this rate.

          why we could just look after the place we have so dumb to think we need to galavant all over the Galaxy as if we a starship with warp drive and find a new place the one we have is awesome just need to get people to stop killing each other and work on keeping this one going.

          Last edited 27/07/15 10:23 pm

            Your view is a bit like telling early explorers not to jump on their sail boats and see whats over the horizon. We'd still think the earth was flat.
            We need to explore beyond our solar system. Who knows what's out there to be found!

        NASA is already working on a WARP drive that bends space-time to achieve propulsion. Potential top speed of this technology could be faster than light. They won't know for sure until it is built and tested in space.

          Yep... smart people are working on teleportation too... doesn't mean it's gonna happen.

          My conspiracy theory is that governments and multi-nationals like promoting these utopian dreams so people don't focus on the fact that we're rapidly screwing the only place we know is 100% guaranteed to support life. It would be about a billion times easier to build a mega city on Antarctica than Mars but like I say... it's good to dream.

          Last edited 28/07/15 8:01 am

            So the plot to Interstellar?

              Is it? I knew I shoulda been a script writer!!!

                Ha really? You haven't seen it? Go watch it man. Fantastic movie.

        Not to mention the fact that the light that we are seeing from this planet to be viewed in our telescopes left the planet 1400 years ago, realistically that planet might have had significant changes since then and could be totally different

      They seem to do that everytime any planet is announced as being located in the "goldilocks zone". OMG EARTH 2.0!

      The media just loves hamming up these stories to get people to read them.

      Like how the Higgs boson became "The God Particle"

      Have people like you actually EVER thought perhaps you're ignorant to someone else's perspective? It's almost you're trying to find bullshit so you feel better than someone. Whenever a story comes out, saying something mildly interesting to which a bunch of people retweet or just shrug and say "cool", you have an equal number of morons coming out of the woodwork telling them the "truth" which is basically a glorified "not exactly" over something people never took seriously in the first place.

      All these people here are talking as if people were like "let's all go tomorrow!" when a grand total of no-one thought this. The very first article I read on it made a massive deal of the expansive and insurmountable distance. There's nothing dumb here but a bunch of insecure guys getting upset over what they incorrectly perceive others to think.

      Last edited 29/07/15 3:36 pm

    A gas-planet like Neptune in the habitable zone would clearly not host oceans since it has no surface. The habitable zone is best considered as a way of narrowing down candidates for investigation in future missions.

    this article is rubbish keep your uninformed comments and opinions to yourself. A GAS planet would NOT exist in the habitable zone because its GAS, you seem not to understand the very basic fundamentals of understanding this. let me guess you did not even google it did you?

    Its called the GOLDILOCKS zone where its NOT too HOT and its NOT too COLD< its warm and perfect for life and other substances to behave similar as they do on earth, ie oceans and frozen poles........

    someone really should go watch COSMOS..........

      Speak for yourself. There are a load of gaseous compounds which are gases at room temperature (roughly 22 degrees Celsius, which is the average temperature of Earth), you twit.

        If incubus knew anything at all about the subject, he'd know that gas planets can and do exist in the habitable zones of stars, and he'd know all about the dozens of "hot Jupiters" and "hot Neptunes" that have already been detected. Ironically, there is plenty of information about these objects just a Google search away.

    no moon means no waves or wind, no neighboring planets means only option is living underground unless you like playing dodgeball with meteorites

    Last edited 27/07/15 6:02 pm

      No-one at NASA said Kepler -452b is definitely alone.

      There could well be other planets in that system that just cannot be detected with current technology.

      As for moons around Kepler -452b they could exist bit definitely cannot be detected with existing technology.

    Damn you GIZMODO. I will now have to cancel the delivery of my Earth 2 TV Series DVDs.

    This article seems overly pessimistic. It is breaking down some assumptions, which were identified as assumptions in the press conference, that are entirely logical. We are fortunate that this planet's orbit lines up between us and its star. There are infinitely more planets orbiting their stars who's orbits are anywhere from full perpendicular to slightly off the orbit tilt that would be required to detect them through the star light dimming technique. Furthermore, the media is looking to spur public interest to maintain/expand funding for this sort of thing. If we don't know where the planets are, then why would we develop the technology to visit them? From necessity comes invention.

    The bit about a carbon planet having a diamond crust and therefor could never change is entirely ignorant of material science. Diamond is not the only solid phase of carbon and it is certainly not permanent, nor is any phase of any material. Except adamantium...obviously.

    The puzzle piece missing from Incubus' comment, and from your rips on him/her, is that the habitable zone is specifically attuned to the phase changes of water at near-Earth pressures on a rocky planet. The pertinent assumption behind these phase changes is that the primary source of thermal energy on a given planet's surface is thermal energy from its star, hence the range variability of the habitable zone from star to star.

      "This article seems overly pessimistic", agreed.
      We are dealing with hard core science here, something the majority don't fully understand, and thats not a problem at all, but we need to communicate it in "layman's" terms somehow. As @MattSci22 said, if the public doesn't understand the importance of at least searching, whats the point. We may as well curl up on our little rock and wait for the inevitable extinction of its life. We always seem to live thinking that our generation is the smartest, we naturally want to think we are the pinnacle of humanity, but we are of coarse not. Next year new discoveries will be made, and many more after we all die, (using historical prediction can be really easy!) our universe still has many secrets to reveal.

      Lets not forget the numbers...we have searched only a fraction of the stars in our galaxy, there are 200-400 billion of them. The estimate is 2.5 billion planets "similar" to earth. Ah yeah, one down, 2.5 billion to go.
      Regards

      Last edited 31/07/15 2:55 pm

    "It is breaking down some assumptions, which were identified as assumptions in the press conference, that are entirely logical."

    Seems as if it's being misleading over the extent to which people were misled.

    Can you guys, like, be reasonable for once? We get you like your "here's why" articles but could you just maintain a semblance of dignity in your writing. Shouldn't be able to destroy an argument's merit by acknowledging information that was intentionally omitted.

    Last edited 29/07/15 3:43 pm

    Be careful in the goldilocks zone. Someone or something just might EAT your porrage.

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