Planets large and small: the Kepler planetary candidates in my TED Talk

Dimitar Sasselov, Co-Investigator, Kepler Science Team, Harvard Smithsonian Center for Astrophysics

Two weeks ago I gave a talk at TED Global 2010 which was very well received, but caused confusion. I referred to past results from the NASA Kepler mission. Indeed, Kepler has not discovered Earth-like planets in habitable zones. We have not found Earth-size planets; at this time we have found only planet candidates – 706 of them as of June 15, 2010, based on only 43 days of data with 306 released and discussed in a paper by the Kepler team. Planet candidates are just that: “candidates”. A sizable fraction will turn out not to be planets, and we do not know what that fraction is yet.

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.

The Kepler mission is designed to discover Earth-size planets by detecting and measuring their transits. The Kepler team collects additional information as it works to confirm a planet discovery, but one essential physical parameter Kepler provides is SIZE, the planetary radius. However, is “Earth-size” the same as “Earth-like”? And vice versa?

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 TED Global conference is about ideas worth spreading to a general audience. In my TED talk I decided to discuss the idea that science seems to be at the threshold of a new revolution – of synthetic biology. Ironically, this is happening at the completion of the previous, Copernican, revolution. My point was that the two events are related, and that, tantalizingly, progress in synthetic biology may be accelerated by input from planetary science. This is the core of the project I lead and we call the Origins of Life Initiative (not associated with the Kepler Mission).

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.

Related links:

TEDGlobal Lecture
Dimitar Sasselov
: “On Completing the Copernican Revolution

Characteristics of Kepler Planetary Candidates Based on the First Data Set: The Majority are Found to be Neptune-Size and Smaller
Borucki, W. and the Kepler Team, submitted to Astrophysical Journal

Claims of 100 Earth-Like Planets Not True
by Clara Moskowitz @

Cosmic Log on
Millions of Earths? Talk causes a stir
by Alan Boyle 

11 thoughts on “Planets large and small: the Kepler planetary candidates in my TED Talk”

  1. Oh, you used the Drake Equation for that one hundred million figure?

    And here I thought that was based on your data. At least, it was strongly implied in the speech that it was based on data. That got a lot of people excited, you know.

    Isn’t the whole point of the Drake Equation to illustrate that we can’t make such estimates because of all the missing data?

    If you actually use it, you’re just converting your biases into a number with false legitimacy.

    Did you just take the number of stars in the galaxy and divide it by a thousand?

  2. hi my name i’m Daniel graduate student of Physics in Brazil,I’d like ask a question i’ll be gladly if could answer me,could be by my email if you wish,please

    my question is: what about the 3000 red dwarf stars(M0-M2)kepler field? in 43 data,did kepler found any earth-size planet candidate in the red dwarf habitable zone?

    thank you

  3. It is ironic that NASA holds on to its most promising Kepler Mission data based on the argument that its release might result in confusion in the media and misinterpretation of the results. Yet that seems to be exactly what NASA now has. Since this is already the case, I suggest all the data be released now.

  4. Joe Cooper, there is no way Kepler could have found 100 million planets or even planet candidates by itself. The field of view is modest-sized and fixed on the sky, and there just aren’t that many stars in it. The number 100M had to have come from extrapolation of Kepler statistics.

    The 100+ candidates found in the first 43 days yield a “large” ratio of small planets to giant planets, in a narrow set of conditions. When extrapolated to longer periods and to all stars in the Milky Way, we can easily justify estimates like 100M, without having to say “Drake equation”. Before Kepler, the prevalence of giant planets was known from radial velocity and other planet searches, but our only information on this small/large planet ratio was from our own solar system. Now we can feel much more confident it’s like that throughout the Galaxy.

    The remaining uncertainties are significant, including verification of these candidates, discovery and verification of longer-period small planets to be found later, the selection bias of transit observations, and many other effects. Even so, the present information justifies the assertion that Earth-size planets can’t plausibly be a factor 100 less common than in our own solar system (for example), because the Kepler sample is close to the same as our solar system.

  5. The moment I saw the Ted presentation, I knew Sasselov was lying and that’s what he was doing–in exchange for applause and looking good, he lied. There was no misunderstanding. He made up data and presented it as fact. Harvard should fire him as should NASA.

    I am tired of scientists misrepresenting science and lying about data and walking away with half-apologies that accept no responsibility–saying something “created a misunderstanding” is to take the blame from the presenter and put it on the audience. This wasn’t an apology, it was a long winded excuse.

    Sasselov traded scientific integrity for applause. He should be openly shamed.

  6. The diagram shown on the lecture TED is the distribution of candidates for the stars brighter than 14 magnitude?

  7. TED’s reputation as a cult-like, self-congratulatory, pseudo-intellectual circle jerk is only furthered by attention-starved clowns like this. If TED is so cutting edge, what was Annie Lennox doing there?;)

  8. Glenn (and anyone else who isn’t clear on why some of the Kepler data was held back for a few months), the reason you give in your post is not why the data was held back from Kepler. The Kepler team asked for an extension since, with the launch date delayed by several months, they couldn’t follow up the candidate planets from the ground during the summer observing system like they originally would have been able to. In order for the Kepler team to have the first crack at the data, which is in the spirit of most missions to allow the original team the first opportunity, the extension simply had to be granted. If the launch of Kepler hadn’t been delayed, the Kepler team wouldn’t have asked for the extension.

    And Joe, Sasselov didn’t lie. If you look at the paper published in the June 15th Kepler release, all the information is there for everything that Sasselov said. All he did was extrapolate from the statistics we have to make a general statement about the galaxy. And truly this whole issue is a mix-up in how you define terms. Earth-like means different things to different groups of scientists as I myself have experienced working in the exoplanet field.

  9. So in proto-planetary discs, some of the planets as a function of their low gravity and their radiation exposure from stars, lose their hydrogen and helium to space and thus becomes “rocky planets” while gas giants are planets who’s gravity are so large or the radiation received is so low that it won’t lose/only lose a little of their light gasses to space

  10. This is a exciting source of knowledge, Im glad I read this article. I am going to be back again soon to see more that you have.

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