Philosophical Society of Washington

Minutes of the 2379th Meeting

At the Cosmos Club, Washington, DC
May 12, 2017

President Larry Millstein called the 2379th meeting of the Society to order at 7:50 p.m.  This was the 86th Joseph Henry Lecture, named in honor of the Society’s first and longest serving president.  President Millstein announced the order of business and welcomed new members.  The minutes of the previous meeting were read and approved.  President Millstein then introduced the four speakers for the evening, Debra Fischer, Hannah Wakeford, Shawn Domagal Goldman, and Aki Roberge.  The lecture was titled “Exoplanets and the Search for Life: An Exploration in Four Lectures”.

Dr. Fischer’s lecture was titled “A Universe of Exoplanets”. She presented the famous “Pale Blue Dot” picture from the Voyager spacecraft, and explained that the goal of exoplanet research is to get a picture of other exoplanets as good as that pixel.

Dr. Fischer described how her team uses the Doppler shift of light from distant stars to deduce the presence of exoplanets that we cannot yet resolve with existing telescopes.  Although planets orbit stars, stars themselves also move slightly, orbiting the center of mass defined by the star/planet system.  This motion of the star subtly shifts the spectrum of light coming from it, and the absorption lines within that spectrum.  The magnitude of this deflection can hint at the mass of orbiting planets.

Dr. Fisher also described the transit technique of detecting distant planets.  By measuring the brightness of a star and noting recurring dips in the brightness of the star, we can deduce the radius of orbiting planets that pass in front of it, which provides their volume.  Together, the doppler and transit techniques can provide the density of the planet, to further discern whether it is a gas giant or small rocky planet of the type that we think may support life.

Dr. Fischer concluded by explaining that the next step for her research is the deployment of the EXPRES “Extreme Precision” spectrometer, with the goal of further refining the doppler method to sensitivities of 10 cm per second, which is enough to detect gravitational disturbances by planets the size of Earth.

Next, Dr. Hannah Wakeford discussed “Exploring the TRAPPIST System with Hubble and the JWST”.  She explained that each element has a unique spectral fingerprint, so that when light from distant stars passes through the atmosphere of a star-transiting exoplanet, not only is the planet detectable, but its atmosphere tints the starlight.  Measurements from the Hubble can detect this tint and provide an estimate of the atmospheric composition of the exoplanet.

Large gas giants are easier to analyze with this method because there is more atmosphere and thus a larger signature.  To analyze smaller rocky exoplanets requires finding planets orbiting colder, redder stars, which make the small planets easier to notice.  The TRAPPIST-1 system provides a superb example of such an arrangement, with a nearby ultra-cool M dwarf star orbited by seven rocky planets.

Dr. Wakefield concluded by noting that the TRAPPIST-1 observations from the JWST will provide us the first solid data to speculate on how easily solar systems and planets like our own form, and what sort of environments we can expect on those planets.

Next, Shawn Domagal-Goldman’s described “The Search for Life on Exoplanets”. Dr. Domagal-Goldman contrasted the ease of finding signs of life—on Mars, and in Earth’s fossil record—with the difficulty of eliminating the false positives, of which there have been many.  Spectral analysis is also prone to false positives, in which telltale elements such as oxygen, methane, or even water can exist on a planet that is nonetheless not conducive to life. The ratio of these gases is key, because life provides a source of oxygen and methane to replenish them and maintain them in an otherwise untenable balance. Discovery of a planet maintaining a high concentration of oxygen in the presence of methane would be, he explained, cause for Champagne.
Finally, Dr. Aki Roberge described “Finding Earth 2.0: Extraordinary Tools to Expand the Search Space”.  She emphasized the need to be able to image planets directly, rather than mixed with star photons, as occurs in the transit method.  Of course, even nearby stars are so distant that the planets orbiting them will be nearly swallowed up in the star’s glare, making direct imaging very difficult.

One answer to this need is coronagraphs, which block out the sun to make nearby planets visible. Internal coronagraphs already exist, which can modify the stream of light entering the telescope. More ambitious “starshade” satellites would attempt to block the light entering space telescopes. Such vehicles would be extraordinarily difficult to build, position, and maintain, but could deliver equally extraordinary imaging capabilities.

The forthcoming WFIRST 2.4 meter telescope is comparable in size to the Hubble and will include a technology demonstrator of a coronagraph and deformable mirror.  It may also be paired with an experimental starshade later in its life.  Ultimately, however, the WFIRST’s lens will be too small to directly image exoplanets.

The true successor to Hubble will be the Large UV Optical Infrared Surveyor (LUVOIR), an 8-16 m aperture, multi-spectrum, upgradable science platform capable of spectroscopically surveying small habitable zone planets, around a range of stars including sun-like stars. Dr. Roberge explained that the telescopes now on the drawing board provide us a very real prospect of finding and describing the first Earth-like exoplanets.

After the conclusion of the talks, President Millstein invited questions from the audience.

In response to a question about detecting other life-friendly attributes, such as moons, Dr. Roberge and Dr. Fischer noted that our theories on what factors are conducive to life are woefully short on data, and have routinely been invalidated by new observations. As Dr. Wakeford put it, our imagination is not as good as nature’s imagination.

Another questioner asked the panelists to share what for them would be a definitive indicator of life. Dr. Domagal-Goldman reiterated that oxygen and methane coexisting together would be a clear watershed. Dr. Roberge emphasized the need for a holistic measure of all of the elements present on the planet, enough to support a strong model of their generation and sink rates. The reliable indicator for life would be an anomaly in these ratios that is not attributable to simple physical or chemical processes.

A final questioner asked the panel to describe their reasons for the search for life beyond Earth. Dr. Wakeford and Dr. Clampin explained that the pursuit of one of mankind’s questions is a powerful inspiration that attracts new generations into STEM and pushes scientific inquiry and engineering achievement forward.  Dr. Roberge added that, if humanity is not to end when the Earth does, we need to already be looking for new homes.

After the question and answer period, President Millstein thanked the speaker, made the usual housekeeping announcements, and invited guests to join the Society.  At 10:04 p.m., President Millstein adjourned the 2379th meeting of the Society to the social hour.

Attendance: 139
The weather: Light rain
The temperature: 10°C

Respectfully submitted,

Preston Thomas
Recording Secretary

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