A World of Difference between 'Earth-Like' and 'Earth-Sized'
But then MSM took things out of context and exaggerated just a tad.
Even though in his talk, Sasselov used the words "potential" and "candidates" and said the planets are "like Earth, that is, having a radius smaller than twice Earth's radius," MSM reported news that NASA has found rocky planets with land and water.
And now some people are saying that Sasselov "leaked" the proprietary Kepler data, and some say he is in trouble for doing so. The Kepler team said via Twitter that they are "working hard to thoughtfully respond to the media flurry surrounding the TEDGlobal talk."
Let me use one of my mother's favorite admonitions: For Pete's sakes!
Watch the TED talk. In my opinion, Sasselov does a good job of getting people excited about exoplanets and he doesn't say we have actually found another Earth. He also does a good job of presenting what the Kepler team has found without revealing any really huge proprietary data, even though he used this graph:
But really, this is pretty much what the Kepler team said in June, that they expected half of the 750 planet candidates would turn out not to be planets, and a fair number of those might be Earth-sized. The graph takes into account the amount of potential planets that Kepler found, plus the planets found previously by other telescopes and missions.
While it is exciting to think about the potential of finding Earth-sized and maybe even Earth-like planets, we're likely a long way off from actually finding and then actually confirming another Earth. Additionally, right now, we're only capable of finding planets that orbit relatively close to their parent star, which most likely wouldn't put them in the "Goldilocks Zone" of being habitable.
So that was my challenge - Kepler measures planet sizes, while I wanted to talk about geochemistry. In just 18 minutes. So, the expected number of planets, size and Earth-like chemistry got confused, and created a misunderstanding.
The family of our Solar System planets seems simple when sorted by size: half of the planets are large (giants) and half of the planets are small (terrestrial). The giants contain a lot of light gases (hydrogen and helium) in their bulk composition while the terrestrial ones contain mostly heavier elements. Too much hydrogen and helium dilute the surface chemistry, while heavy elements and solid surfaces tend to concentrate it. There is one planet in our Solar System where the chemistry has evolved to biochemistry and to a biosphere. In the search for life beyond Earth, the smaller planets are thus the favorite places to look.
Kepler is capable of finding Earth-size planets in orbits of moderate temperatures. But most people consider the term “Earth-like” to mean that the planet has an atmosphere, liquid water on its surface, and a temperature conducive to life. In other words, "Earth-like" is often used to mean 'habitable'. Therefore, Earth-size and Earth-like are certainly not the same. Take the example of Venus, an Earth-size planet whose surface will melt lead.
The term "Earth-like" planet creates confusion. To some scientists like me, who model planet interiors, the term "Earth-like" is a simple short-hand for a bulk composition like Earth's. It emphasizes the broad difference between gas giants and terrestrial planets, as seen in our Solar System. However, I understand that this is not how it was interpreted by the majority of the media coverage. My definition allows for a whole range of planet sizes to be "Earth-like" planets. Thus, the question - what size planets might be "Earth-like"?, is more interesting. According to my definition, it involves the so-called "super-Earths" - planets larger in size and mass than the Earth, yet smaller than the giant planets. Many super-Earths are expected to have the same properties and potential for life as habitable Earth-size planets.
Kepler planetary candidates, like the 306 released this past June 15th, have estimated orbits and sizes. Sorted by apparent size, the majority of the candidates are found to be Neptune-size and smaller. This is the good news. As of today none of the candidates smaller than 2 Earth radii is in the habitable zone; their orbits are too small, which is why it was easier to spot them after just 43 days. Habitable planets will take a lot more time, as Kepler needs to observe more than one transit.
The first data release is an encouraging first step along the road to Kepler's ultimate goals, specifically, to determine the frequency of Earth-size planets in and near the habitable zone. However, these are candidates, not systems that have been verified sufficiently to be considered as planets. The distribution of planet sizes may also change. It will take more years of hard work to get to our goal, but we can do it.
The Origins of Life Initiative makes connections between geochemistry and biochemistry. The Kepler mission helps our project by establishing feasibility: if solid planets are common, then we have a shot at trying our lab experiments. This works as long as the planets have geochemical cycles that determine their atmospheric signatures. Hence, planets somewhat larger than Earth are more favorable (Sasselov & Valencia, Sci. American, Aug 2010). The Drake equation gives us an estimate of about 100 million such planets with habitable potential in the Milky Way galaxy. Kepler has not yet weighed in on this yet, but holds the potential to do so in the future.