Last night, I headed down to the beautiful Bell House, in the Gowanus section of Brooklyn, for this month's Secret Science Club lecture, with Princeton University professor of aerospace engineering Dr Jeremy Kasdin and principal investigator of the Exo-Starshade Project. Dr Kasdin's lecture concerned the search for extrasolar rocky planets, earthlike planets orbiting other stars.
Dr Kasdin began his lecture by asking what are the challenges involved in detecting distant rocky planets, why is it so hard? In April 2014, the first earthlike planet was detected in the habitable zone of a distant star by astronomers using the Kepler Space Telescope. The ability of a planet to sustain life is thought to depend on the presence of liquid water.
The discovery of exoplanets has dramatically increased over the past year. There are various methods of detecting exoplanets- radial velocity employs changes in the velocity of a planet as it moves toward or away from Earth that result from the gravitational effects of orbiting planets. By 2003, all of the extrasolar planets that had been discovered were all Jupiter-sized giants close to the their stars. In order to find smaller planets, there was a need to go into space.
Planets can also be found by imaging, seeing planets. They can be detected by transit, when a planet passes in front of a star, it blocks light from the star, resulting in a diminution of the star's brightness. The size of a planet can be determined by the amount of light that it blocks. The further from a star that a planet is, the longer an orbital period it has, so the more time is needed to detect it.
The first exoplanets were discovered using the radial velocity method. The first planet detected using transit was HD 209458 b- the use of transit confirmed a detection via radial velocity. Radial velocity and transits have been the most successful exoplanet detection methods so far.
The Kepler Space Telescope was designed to detect small, rocky planets by counting photons in order to measure planetary transits. Over one hundred thousand stars were targeted by Kepler and thousands of planetary candidates, mainly small planets not in the habitable zone, were discovered but not confirmed- in order to confirm the presence of a planet, three transits must be observed, but an equipment failure after three years led to a failure to confirm. In 2013, Astrophysicist Francois Fressin corrected for biases in order to clean up data accumulated by Kepler and announced that earthlike planets were common- the majority of planets are small, though the big planets were discovered earlier. It seems that the two most prevalent types of planets could be described as Super-Earths or Small-Neptunes. Probing the outer reaches of other solar systems could help determine the composition of exoplanetary atmospheres.
The initial discovery of exoplanets was through indirect observation- one would look at a star and extrapolate the presence of planets orbiting it. Direct observation of exoplanets is now sought. With the evidence accumulated by Kepler, it is now thought that every star has at least two planets. Recently, direct observation resulted in the discovery of a giant planet orbiting the star Fomalhaut, along with a huge debris disk. Similarly, four giant planets were detected around the star HR 8799 through direct observation.
One technique for determining how earthlike a planet is would be to search for earthshine, an atmospheric combination of water, oxygen, and carbon dioxide that mimics Earth's atmosphere- figure out what light is absorbed by the atmosphere of a planet, and compare it to the Earth's atmosphere. In particular, absorption of lightwaves in the 700 nanometer range would suggest the presence of plantlike life. Plants also emit infrared light. The first exoplanet for which a spectrum was obtained was HR 8799. In order to have more success in capturing spectrographic data for exoplanets, an orbiting observatory is needed.
Dr Kasdin then posed the question, why is imaging so hard? He displayed a series of images of Pluto and its moon Charon, with the two objects indistinguishable in certain images due to resolution problems. As light enters a telescope, it is diffracted- the central disks of bright stars appear bigger and diffraction rings form. Planets being so much dimmer than their stars, they tend to get swamped by the diffraction patterns- with the contrast problem, they cannot be distinguished from their planet. Larger telescopes can reduce diffraction because they resolve better. Reflecting telescopes using secondary mirrors held in place by struts cause diffraction spikes.
Diffraction patterns can be fixed in three ways nulling interferometers use secondary optics to cancel out starlight, internal coronagraphs are attachments to telescopes which block out starlight, and external occulters block starlight entering a telescope from a distance. The topic of the talk then shifted to external occulters. Scattered starlight creates visual "noise" called speckle which obscures planets- by blocking the star, the scattered light is reduced and image errors can be corrected. Deformable mirrors can correct wavefront errors. Another way to reduce diffraction is to use shaped pupils to apodize light to reduce diffraction effects. Atmospheric distortions and imperfect optics reduce contrast.
Dr Kasdin then enumerated a number of new exoplanet detecting projects such as the Gemini Planet Imager, WFIRST-AFTA,and as a pièce de résistance, the Exo-Starshade, an external occulter which is designed to create an artificial "eclipse" outside of a telescope to obscure a star in order that its planets can be seen. He then presented lovely images of a proposed sunflower-shaped occluder, a forty-meter diameter apodizing occluder which is supposed to be positioned 40,000 kilometers in front of an orbiting telescope in order to allow planets to be seen. Here is a video of a TED talk that he presented about this occluder:
Pop open a beer and watch it so you can get a taste of the Secret Science Club. The bastard missed much of the Q&A because he needed to answer a call of nature. After the lecture, he asked Dr Kasdin if the discovery of extremophiles stretched the notion of the habitable zone (touché, doctor), but Dr Kasdin wryly noted that all of the extremophiles that we have encountered are firmly in the habitable zone, and that a certain amount of anthropocentrism is inevitable when it comes to searching for life. After the lecture, my friend Ben joked that the habitable zone is where the bank hasn't redlined the neighborhood.
Once again, the Secret Science Club has served up a fantastic lecture, a combination of science fact and the nuts-and-bolts work that underlies scientific discovery Kudos to Dr Kasdin, Margaret and Dorian, and the staff of the beautiful Bell House.