May 12, 2016 | David F. Coppedge

Earth Twin Still Missing in Exoplanet Trove

The Kepler spacecraft has found 2,325 exoplanets so far, but there’s still no place like Earth.

Live Science chose to frame the news optimistically. Its headline reads, “9 New Habitable Zone Planets! Huge Haul of Worlds Found By Space Telescope.” Exclamation point, even. But it takes more than being in the zone to qualify as an Earth twin. Two other news sites show a sad face at the news:

  • 1st Alien Earth Still Elusive Despite Huge Exoplanet Haul (
  • More than 1,000 new exoplanets discovered – but still no Earth twin (Andrew Norton in The Conversation)

To keep hope alive, optimists say Kepler is not done yet (it may work into 2018). Sooner or later we’ll get lucky, Andrew Norton says:

The latest announcement is an impressive piece of work, and the discovery of so many new exoplanets is stunning. It is increasingly clear that planets orbit stars as a rule – not an exception. While astronomers still haven’t found an exact twin of the Earth, the rapid pace of discoveries is surely a sign that it is just a matter of time until they do.

Maybe they will. But a million planets would not be encouraging if they are like Venus, Mars, or Mercury. Look at our moon; it is in the habitable zone, and astronauts have shown you can walk there. But we all know what would happen if you took your helmet off. Norton sifts through the new data:

From the newly identified sample [of 1,284 new exoplanets], around 550 are smaller than twice the radius of the Earth, which means they could be rocky in composition. Nine of these lie in the optimistic habitable zone around their stars. However, six of the nine lie on the extreme inner edge of the habitable zone and another lies on the extreme outer edge. This leaves just two firmly within the “conservative” habitable zone and only one of these – the exoplanet Kepler 1229b – is similar in size to the Earth at 1.1 Earth radii. However, even that is not in an Earth-like orbit, as its parent star is a cool red dwarf which the planet orbits once every 87 days.

Kepler will keep on looking, but it is rather surprising to find virtually none so far that could support life as we know it. Earth represents just 0.04% of the sample at this time. Admittedly it’s a small sample in the big scheme of things. But Norton reminds us that the requirements for an Earth twin are stringent:

Such a planet would have not only a radius similar to the Earth but a mass similar to the Earth too (and so presumably a similar bulk composition) and it would orbit a star of similar mass, size, luminosity and temperature to the sun in an orbit that takes around a year to complete.

Orbital Woes

News posted on Science Daily says that Earth dodged an orbital bullet, compared to a system of four planets orbiting Kepler-223. All much larger than Earth, they got locked into fragile resonances that they’ve maintained longer than expected. This calls into question how Earth survived the gravitational billiard games other planetary systems seem to have endured. Lead author of a new paper confesses, “Exactly how and where planets form is an outstanding question in planetary science.”

Oxygen Woes

Oxygen is life-giving, except when astrobiologists don’t want it. Oxygen destroys the molecules they want for life to emerge in the fabled prebiotic soup. A paper in Nature now claims that oxygen appeared on the early Earth much sooner than thought — 2.7 billion years ago, instead of 2.4 billion, a difference of 300 million years. And if the meteorite data on which this conclusion is based isn’t refuted, it also means the Earth had almost as much oxygen then as it does now. “Shooting stars show Earth had oxygen eons before we thought,” Jeff Hecht says on New Scientist.

We were very surprised to find micrometeorites at all, let alone those with iron oxides,” says Matthew Genge of Imperial College London. “It was incredible, these tiny spherules had trapped ancient atmosphere, storing it away like little treasure chests.”

The biggest surprise was the presence of oxygen, says lead author Andrew Tomkins of Monash University in Australia. “As geologists, we are taught the Earth had no oxygen in its atmosphere before 2.3 to 2.4 billion years ago.

One NASA scientist remarked, “It is remarkable that objects as small as the micrometeorites survived intact for 2.7 billion years.” Given this surprise, can geologists claim any time when the Earth lacked oxygen?

If oxygen goes back to the beginning of Earth, they can kiss their little OOLS good-bye (origin-of-life scenarios).

What can we safely conclude from these three reports? Earth looks designed for life, just like Isaiah said.


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  • John C says:

    What truly amazes me is their surprise at finding oxygen in an iron oxide deposit. What do these people think oxides are?

  • Michael says:

    A big thanks to you David, for your perseverance and hard work with this site.

    A question for you and your readers, which do you think would be a bigger blow to the astrobiologists, earth like planets remaining undiscovered, or, the discovery of earth like planets but no intelligent life?

    I think that the latter would be a bigger blow.

  • lux113 says:

    Michael, I have to wonder if one day we will find a planet that fits the bill perfectly.

    It’s impossible to guess what God has planned. I believe that God needed there to be a somewhat plausible materialist explanation for the universe (well, plausible to them anyway). I know that some could see that as a form of deception on his part – I truly do not, I see it as essential for his plan. It’s essential for people to have the choice of God or no God, and there needed to be a way to rationalize the no God option for our faith to have real worth.

    I often wonder how many things would effect the believing world. What if we found aliens? What if we found zombies? Or if an asteroid hit us and wiped out a large part of the population… ?

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