Smaller Planets Rule the Galaxy

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Like Indiana Jones opening a new vault of buried treasure, the much-awaited first year treasure trove of NASA Kepler space telescope observations of distant planets went public last week.

Or at least some of the data have, as reported by Nicole Gugliucci regarding the dilemma of long-sought exoplanet detections being released to everyone after a one-year propriety period.

WIDE ANGLE: The Age of the Exoplanet

Several science papers from the Kepler team were immediately posted on the Internet, and will be published in journals once peer-reviewed. The targets are cautiously called “planet candidates” until follow-up dynamical observations can be done from ground-based telescopes.

Nevertheless, Kepler’s first harvest provides an entirely new vista on the exoplanet landscape. This is thanks to Kepler’s unique ability — enabled by being located in space — to detect comparatively small planets passing in front of (transiting) their stars. This produces as very slight dimming of light (a fraction of a percent) coming from the star.  The dip is only clearly measurable from above Earth’s atmosphere for the deep starfield sampled by Kepler. 

Prior to the Kepler data release, the over 450 exoplanet discoveries have been lopsided by the limitation of our detection capabilities. Before Kepler the large majority of exoplanet detections were made through measuring the wobble of a star as it is tugged by an unseen companion, like a small dog on a leash. (Several small planets have been detected through gravitational microlensing, but follow-up observations are impractical.)

This unexpectedly led astronomers into the vault of the “hot Jupiters,” massive migrating planet that snuggle up so close to their stars such that they complete orbits in just a few days — or even hours. Astronomers picked the lowest hanging fruit: fast moving planets that have an easily measurable tug on their stars.

Kepler is opening a completely new vault: collecting a survey sample of small planets that transit their stars. The Kepler results show that there are lots and lots of smaller worlds, ranging in mass from the size of Neptune down to “super-Earths” which are just a few Earth masses.

This is an encouraging major step along the Yellow Brick Road to the ultimate destination: an Earth mass planet in the habitable zone of a sun-like star. That payoff is at least two years away because a true Earth analog would only transit its star once a year. Several annual transit observations are needed to provide convincing evidence that a suspected Earth analog has really been found.

But in the meantime, Kepler has delivered compelling evidence for at least 300 comparatively small planets that orbit inside the inner edge of the habitable zone around their stars. These are low hanging fruit too because their swift orbits are completed within a matter of days. Therefore, it’s easy to collect enough repeat transits to convince astronomers they are seeing the real thing.

Kepler also identified several systems where more the one planet was seen transiting the central star. This greatly bolsters the common idea that systems should largely be coplanar (with some exceptions) because the planet formed in a flattened gas and dust disk encircling the newborn star.

The bottom line is that smaller planets are more numerous than big planets. This bodes well for finding habitats for extraterrestrial life as we know it. It is also encouraging that candidate Earth and super-Earths should, statistically, be abundant in our solar neighborhood.

But detailed spectral “sniffing” of atmospheres for signs of life can be accomplished only within a radius of about 100 light-years from us. And the search for those nearby worlds is already underway from a ground-based survey of 2000 nearby red dwarf stars.

It will also require a future space-based photometric transit survey telescope called the Transiting Exoplanet Survey Satellite (TESS), to carry on the quest for a nearby Earth cousin. These will be targets to look for the biosignatures of life by the upcoming James Webb Space Telescope.

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