In the nearly 20 years since Michel Mayer and Didier Queloz confirmed the first known exoplanet around 51 Pegasi, the number of known exoplanets has risen to over 1860 – and there are more to come. The Kepler space telescope, before being hobbled by mechanical failure, created a massive database of planet candidates orbiting the 150,000 stars it looked at – some 4,175 in fact – which are still being checked. Eight new planets were confirmed just last week.
We can be sure there are a lot more out there. Kepler scanned just 0.28 percent of the sky in the direction of Draco, out to 3000 light years. In that patch, it could only detect planets whose orbits cross the disk of their star from our viewpoint. Other planetary systems, tilted at different angles, aren’t detectable by the transit method. But they will be there. And now the hot question – how many planets are like Earth?
Astronomers have found a few planets Earth-sized and below – including two of last week’s confirmations. Some are in the ‘Goldilocks’ orbit where the star’s warmth would allow liquid water to flow on a planetary surface. Though bear in mind that an observer using Kepler to scan our solar system would classify Venus as “Earth-sized” in the habitable zone. The problem is that transit-detection gives us diameter and orbital period, hence mass and density of the planet (and of its parent star). But it doesn’t give visual data – we can’t do spectroscopy on the atmosphere, for instance, though that’s possible with other techniques, and some data has been fielded about planetary atmospheres.
However, it’s only a matter of time (and money) before instruments are able to pick up more data from subtle fluctuations of stellar light. A photon here, a photon there – literally. From that, we’ll learn about planetary colour, atmospheric composition (via changes to starlight passing through it). Maybe we’ll learn whether any have large moons, if the orbit of that moon is in line with the star. Though I wonder. We’re looking for another Earth – but who says our world has been replicated?
One of the types we’ve found is the ‘hot Neptune’ – a world maybe twice the diameter of Earth with eight or more times the mass. About 19.3 percent of exoplanets found so far fall into this category, as opposed to 5.3 percent of Earth-sized worlds. They also orbit relatively closely to their stars. This is largely a function of technical limits – we can detect the bigger worlds more easily, and picking up the orbits of worlds that are distant from their stars requires years-long observations. So these proportions will likely change. But for the moment that’s where the data points.
Close to its primary, such worlds could be water planets, rather than the ice giants we have in our solar system. Maybe these ‘exo-Neptunes’ define ‘normal’. Or maybe every world is unique – product of many variables, obeying the same laws of physics but emerging in variations defined by subtle differences in composition, size, ambient temperature, and so on. Check out Jupiter’s biggest moons – all different, all formed in the same place at the same time.
The realities of physics mean we won’t travel to these exo-worlds any time soon. Or later (and yes, I know about the ‘Alcubierre drive’). But it’s fun to speculate…and I have a question. Suppose we found another Earth and arrived, en masse. Do you think we’d ruin it, the way we’re making a good job of ruining the Earth we’ve got? Just wondering…
Copyright © Matthew Wright 2015