The Astronomers who cried Earth!

Since the first discoveries of planets outside of our own Solar System in the early 1990s, the quest has been on to find a “second Earth”. The number of candidate extrasolar planets detected has rocketed in recent years, thanks to dedicated missions and telescopes, such as WASP on the ground and Kepler in space. Today the number of exoplanet candidate detections stands at over 700, all found within my (25 year) lifetime.

This is obviously an exciting field in astronomy, both for the scientists actively working to discover and understand these exoplanets and for the public who, in a way, are seeing science fiction become reality (for some reason the quest for Earth in new Battlestar Galactica always comes to mind!). However, I do feel that some of the press releases (and subsequent news stories) that accompany these discoveries are clutching at straws slightly.

There seem to be two categories of extrasolar planets that the media love. One is the “Earth-like” planets. These are planets that astronomers believe to some characteristic that is similar to the Earth. It usually means that they are similar in size (i.e. radius) or mass. However, this can be deceiving. For example, the planet could be orbiting a pulsar or it could have a surface temperature of -200 degrees celsius! Earth-like doesn’t necessarily mean that we could live there. The second category of planets that get the media excited is the habitable, or Goldilocks, zone planets. A planet in the habitable zone is basically orbiting its star at the right distance for liquid water to exist on the surface. However, there’s nothing to say that these planets have to be rocky like the Earth, and gas planets have been found in habitable zones around stars. Of course, this is still interesting, especially if they have rocky moons (like Saturn’s moon Titan), but again, it’s something to bear in mind.

The first press release I could find claiming the discovery of an Earth-like planet was from ESO in April 2007. This press release announced the discovery of Gliese 581c and was titled “Astronomers Find First Earth-like Planet in Habitable Zone”.Gliese 581c is (unsurprisingly) orbiting around the star Gliese 581, a red dwarf star about 20 lightyears away from us. Because its host star doesn’t output as much energy as the Sun, Gliese 581c is much closer to its star than Earth is to the Sun and only takes about 13 days to complete an orbit. In case you were confused how this might affect the view on Gliese 581c, the Daily Mail helpfully included a cartoon in their report!

The planetary system around Gliese 581 has actually made headlines 3 times for claims of habitable zone planets. Following the discovery of Gliese 581c, in 2010 the discovery of the planet Gliese 581g was reported as “NASA and NSF-Funded Research Finds First Potentially Habitable Exoplanet”. Note the use of language, it’s the first “potentially habitable” planet because by this time, doubts had been cast on the ability of Gliese 581c to support life. However, interestingly, one of the astronomers involved with the study was quoted as saying that it was the first planet in the system to be found in the habitable zone. Since then, further analysis of the data indicates that this planet might not actually exist. Then, earlier this year, there were claims that Gliese 581d could be the “first definitively habitable planet outside our Solar System”. Three planets around the same star, all of which have been claimed to be the first habitable exoplanet. Is anyone else confused?

Then this month there have been two more announcements of Earth-like/habitable planets, both from the Kepler mission. Kepler-22b was annouced as a “Super-Earth in the habitable zone of a Sun-like Star”. This led to the planet being dubbed Earth 2.0, Matt Burleigh has already covered what’s wrong with this. Yesterday there was a press release from NASA stating that “NASA Discovers First Earth-size Planets Beyond Our Solar System”. Kepler-20e and Kepler-20f are similar in size to the Earth but are way too close to their star to host life. So not really like Earth at all.

There have been so many “Earth-like” and habitable zone planets claimed recently that, as my officemate put it this morning, some of us are getting Earth-fatigue. I’m not sure who to blame for this. I would imagine that it’s the respective press offices of the universities or organisations trying to put a spin on the discoveries, and not the astronomers themselves. I guess it is important for us as scientists to let the public know what we are doing, after all they do fund (a lot of) us. Unfortunately this often requires a hook so you’d better hope that what you’ve found/are studying looks like food or is a giant gemstone in space. I just hope that when we do find a planet that is similar to Earth in size, mass, composition, atmosphere and is in the habitable zone, i.e. a true Earth 2.0 that we could live on (provided we could get there!), the public isn’t so sick of us crying wolf that they just don’t care.

And don’t even get me started on the “Tatooine-like” planet…