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All These Worlds

The Kepler spacecraft continues to reap a rich harvest of new planet discoveries.

In February, the Kepler Team made two impressive announcements that give us reason to celebrate and anticipate more from this wonderful spacecraft as it steadily reveals the diversity of planets in our galaxy.

The first is Kepler-11, a bizarre mini-me solar system, with 6 planets huddled so close to their star – a yellow dwarf like our sun – that they would all fit inside the orbit of Venus.  Exoplanets are notoriously hard to scrutinize.  Sometimes I think that, after the euphoria of rapid discovery, the depression will set in as we realize how hard it is to actually know much detail, beyond the sizes and orbital distances revealed by the minuscule dips in light as they pass in front of their star. Yet now, thanks to some reliable physics and clever analysis, we know much more about these six new worlds. As they orbit they gravitationally jostle, like roller derby skaters pushing and pulling one another ahead or behind, causing tiny changes in orbital timing that reveals the mass of each planet.  This gives us modelers a lot more to play with.  Distance from the star gives us temperature. These worlds are smoking hot.  Size and mass yields density, which narrows down what they can be made of.  They are all surprisingly light for their sizes, their low densities revealing puffed out atmospheres.  These sizzling, fluffy hydrogen-rich atmospheres must be unstable, furiously losing gas to space.  Why are they still there at all, or are they just young and full of hot air which will be lost as they age? For those of us trying to understand the evolution of planetary atmospheres, this is juicy stuff.

We can only deduce all this because these planets all orbit in our line of sight as we peer across the galaxy.  Most randomly oriented systems will not display themselves so conveniently for our interstellar voyeurism.  Did we just get lucky? In a sense yes, except that the Kepler is designed to make its own luck by monitoring enough stars, about 155,000, so that the small percentage of planetary systems that would be randomly oriented in our direction will add up, if planets are reasonably commonplace, to a decent demographic slice. This scheme is working, paying off in droves of new discoveries, slowly revealing the planetary demographics of our galaxy.

The Kepler strategy of calculated luck is also reflected in the announcement of 1235 candidate planets discovered during the first 4 months of Kepler science operations, including a handful that are roughly Earth-sized and zoned for liquid water. This is not the same as announcing planet discoveries.  The Kepler method is sure to produce false positives.  Stars vary in brightness, and, especially in such a crowded slice of the galaxy, something appearing close to a star might be changing, or a multiple-star system could mimic a planet discovery.  Each candidate must be verified with repeat observations and, where possible, other instruments and methods.

In science we frequently have a tension about when to release something important.  Wait until you’re sure, or spill the beans when it seems fairly promising?  There are risks either way.  Often there is no magic moment, and with Kepler the process of discovery is stretched out over years.  I’m thrilled with the Kepler team’s decision to announce the statistics of candidates.  Most, but not all, will turn out to be actual new worlds.  Sharing your data, when it is still at this stage of immaturity, assumes some maturity among the public.  It requires some understanding of probability and uncertainty.  This is a crucial part of the mathematical and scientific literacy needed to understand some urgent current issues.  So much of the noisy bruhaha over climate change, for example, comes from misunderstanding or misrepresenting of scientific uncertainty.  So I applaud the the Kepler team for releasing these planetary candidates and expecting people to understand that it will take time to narrow down the uncertainties and know exactly which ones are really planets.  It’s a tease – a respectful tease.