This week the SETI institute announced they were going to check the newly discovered Earth-size world 1400 light years away, Kepler 452b, for radio transmissions. I don’t think they’ll find any. Here’s why.
The problem is that near-Earth size, insolation and orbit – which is all we know just now – doesn’t necessarily mean Earth-like.
The planet was first detected in Kepler space telescope data in May 2014, but it’s taken until now for a team under Jon Jenkins, Joseph Twicken and others to make the careful follow-up observations to confirm it. The way it works is this. Between 13 May 2009 and 11 May 2013, Kepler looked at a patch of sky in the direction of Cygnus, scanning some 111,800 stars on a continuous basis. The detection principle is ingenious. When a planet – even as small as Earth – crosses between the target star and Kepler’s detectors, there’s a wobble in the star’s light curve. Kepler can pick that up. After a while, the planet orbits around and the wobble’s repeated.
That double detection is the key, because it gives the orbital period. Knowing the orbital period allows the distance from the star and the mass of both bodies to be calculated. The light wobble also allows an estimate to be made of the amount of light blocked by the planet, which – once the distance from the star is known – gives the planetary diameter. From planetary diameter and mass it’s possible to calculate density, which allows an estimate to be made of composition. So we know that Kepler 452b has a diameter 60 percent larger than Earth, and five times the mass. That gives it a surface gravity (a function of mass and planetary radius) about double ours. We also know the density, meaning it’s likely to be a rocky world. And the implication is that it’s going to have a hot core – implying volcanoes.
This data also reveals that Kepler 452b orbits about the same distance from its star as Earth from the Sun – its year is 385 days. The star is a shade brighter than the Sun, giving about the same insolation. That means a theoretical surface temperature can be calculated, though it’s complicated because Kepler 425b’s atmospheric composition is unknown. For comparison, Earth’s true average temperature is 255 Kelvin, -18 Celsius or 0 degrees Fahrenheit. We’re warmer than that, by 14 degrees C on average, because carbon dioxide (mostly) produces a greenhouse effect, which acts as a heat trap. Without an atmosphere at all, we’d be like the Moon, soaring to +123 degrees C at high noon and plunging to -183 degrees C at night.
Some extra-solar planets have been observed for long enough, and with enough different methods, to yield details about their atmospheres. When a planet transits its star, it’s sometimes possible to see how the spectrum of the star is distorted by the planet’s atmosphere – and thus work out what gases are in the atmosphere. Sometimes.
Kepler 452b’s actual atmosphere is guesswork at the moment, but the picture of a large planet with dramatic volcanic activity actually points to Venus, where the greenhouse effect went into runaway mode. The outcome? A world with 90 times the surface atmospheric pressure of Earth, sulphuric acid clouds, and surface temperatures that make a souffle oven look cool.
Will we find a more certain match for Earth? Probably, sooner or later. Apart from the 4661 exoplanet ‘candidates’ already identified from Kepler data for follow-up confirmation – of which 1028 have been confirmed so far – Kepler itself only made a partial survey, a slice of sky to 3000 light years in the direction of Cygnus. That’s a tiny proportion of the galaxy. It could also only find objects in a line between its detector and the parent star. The number of planets whose orbital planes, coincidentally, happened to be in that line makes clear that planets are very common indeed. There are bound to be many other systems Kepler couldn’t detect because the planets were orbiting in the wrong plane. It was also not operating long enough to pick up long-period planets (to discover Uranus and confirm its orbit, for example, an alien telescope would need to be looking at our solar system for over 80 years).
To me that says that planetary systems are ubiquitous.
And one more thing. On the data so far – admittedly, likely an artefact of the way we’ve been able to detect extra-solar systems so far – our solar system seems an oddity. We’ve found systems with ‘hot Jupiters’ skidding close to their star. We’ve found a lot of worlds the size of Uranus in Earth-like orbits, (dubbed ‘hot Neptunes’, for some reason). But very few systems seem to be like ours.
To me, that makes our Earth all the more valuable. And the onus is on us not to ruin it.
Copyright © Matthew Wright 2015