Artist's concepts of the Kepler-444 system with five exoplanets. Image credit:​ Tiago Campante/Peter Devine

Jul 27, 2015 New frontiers in exoplanet science

Sen—Many people remember the excitement surrounding the first round of 100 Kepler exoplanet candidates more than five years ago. And many also listened in dismay in 2013, as NASA announced the irreparable failure of two of Kepler’s four reaction wheels (spinning flywheels that allowed precision control over the telescope’s pointing). Though the observatory lasted just over half its intended lifetime (indeed, as the great poet, Billy Joel, hath writ, only the good die young), all is not lost in the search for other habitable—and even hospitable—worlds. NASA designed Kepler to detect exoplanets that were transiting (briefly, but periodically moving in front of) stars within the Milky Way’s disk, which requires extremely accurate and precise telescope pointing, impossible with only two reaction wheels. All seemed lost.

The indomitable exoplanet community, though, refused to go down without a fight, and engineered a method to bootstrap extra stability, by balancing the Sun’s radiation pressure on the observatory’s solar panels (this has been likened to balancing a pencil on its tip.) The new observing paradigm, lovingly christened K2, achieved photometric precision of less than 50 parts per million, a statistic estimated to approach that of the original mission. K2’s first exoplanet transit, the catchily-named HIP 116454, was announced in late 2014. In addition, a 2014 paper published in the Astronomical Journal by the citizen science project PlanetHunters reported the discovery of a seventh planet orbiting the star KOI-351 (the “KOI” stands for “Kepler Object of Interest”), making that star system the most populous discovered to date.

What is more, new computational methods have afforded even better use of the available transit data. A recent paper by Ellen Price of Caltech and collaborators provided a proof of concept for the so-called “photoeccentric effect” (not to be confused with the photoelectric effect). Price notes that exoplanet models have many parameters that conventional methods generally fit simultaneously, including eccentricity, a measure of the orbit’s shape. “Using the photoeccentric effect,” says Price, “lets you fit your light curve to a model as if it was circular (much easier!) and constrain eccentricity after the fact.” Referring to an earlier paper by John Johnson of Caltech and Rebekah Dawson of Berkeley, Price observes that “even a ‘vague’ idea of the stellar density, can help you constrain your eccentricities.” To paraphrase Douglas Adams, you get your orbit fits without all that tedious mucking about in parameter space.

K2’s observations will complemented starting in August 2017 by another NASA mission, the Transiting Exoplanet Survey Satellite (TESS), which will operate very similarly to Kepler. TESS, though, fills a unique niche in both exoplanet period and sensitivity, untouched by either Kepler or K2 (TESS is specifically designed to look for planets orbiting very nearby stars). As a point of interest, NASA has contracted none other than SpaceX to launch TESS in 2017, underscoring the federal government’s growing confidence in the upstart rocket company. Not to be outdone, ESA has announced its own mission, the Characterizing Exoplanet Satellite (CHEOPS), which will follow-up known planets to better constrain their size and orbital period, as well as the stars they orbit.

Exoplanet science is also having an impact on one of the most venerable observatories in the U.S.—Kitt Peak, in Arizona. In 2018, the 3.5-meter WIYN (pronounced “win”) telescope, operated by a consortium including the University of Wisconsin, Indiana University, the National Optical Astronomy Observatory, and the University of Missouri, will welcome a new instrument, the Extreme Precision Doppler Spectrometer (EPDS). In contrast to the transit method employed by Kepler and K2, EPDS will measure the periodic wobbles of stars as they are pulled to and fro by a planet, a process called Doppler spectrometry. To characterize the wobbles, astronomers use precise measurements of a star’s spectral lines to assess its velocity over time. A single planet in orbit will produce a minute sinusoidal variation in a star’s velocity, the amplitude of which will depend on the planet’s mass, relative to its host star. Multiple planets will of course produce more complicated signatures. By studying the spectra as a whole, astronomers first find the mass of the host star (since each mass of star evolves along a largely-distinct path with more or less distinct spectra), and can then deduce the mass of the star. The overall period of the oscillations can then be used, along with the measurement of star’s mass and all of the planets’ masses, to calculate the planets’ orbital radius and deduce whether it lies within the star system’s habitable zone.

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The WIYN building. Image credit:NOAO/AURA/NSF

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The WIYN 3.5 meter telescope. Image credit: NOAO/AURA/NSF

According to the Proposal Solicitation, obtained through NASA’s NSPIRES portal, EPDS will be capable of measuring radial velocities with a precision of 10 to 50 centimeters per second—slower than a leisurely walk. This is a factor of about 10,000 better than WIYN’s current instruments, which are used to study large-scale motions of stars within the Milky Way and populations of stars in other galaxies. Such precision is necessary for measuring the minute wobbles in the star’s position, caused by one or more orbiting planets. EPDS will be used to follow-up exoplanet candidates detected with TESS and other space-based missions, as recommended in the Astro2010 decadal survey, which challenged NASA and the NSF to "support an aggressive program of ground-based high-precision radial velocity surveys of nearby stars in order to validate and characterize exoplanet candidates." In June, two EPDS proposals were selected for support, and according to materials provided at the March 2015 pre-proposal workshop, the down select to one proposal will occur in March 2016, after each group has completed an Instrument Concept Study (ICS).

A third method of exoplanet detection, gravitational microlensing, reads like a sci-fi novel. Einstein’s theory of General Relativity tells us that light can be bent by massive objects, like planets. Every so often, a planet orbiting a distant star will pass in front of another background star, purely by chance. This will produce a quick spike in the amount of light we see from the background star, as the planet acts like a lens. An advantage of microlensing detections is that they largely probe exoplanets in Earth-like orbits (from 1 to 10 Astronomical Units). Unfortunately, microlensing events only occur once for a given star system, they are difficult to follow up, and they give minimal information about the orbit’s shape. Several groups, such as the Optical Gravitational Lensing Experiment (OGLE), have built robotic telescope networks to cheaply survey the sky for such events, and have found planets such as OGLE-2005-BLG-390Lb, a 5.5-solar-mass planet in a 2 to 4 AU orbit.

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Graph showing data obtained from the Optical Gravitational Lensing Experiment (OGLE) and NASA’s Spitzer Space Telescope during a gravitational microlensing event. Image credit: NASA/Caltech

The combination of all the above techniques promises a renaissance in exoplanet science over the coming years. Perhaps it’s only a matter of time until we find Earth 2.0.

The Author

This blog was written by Zach Pace, pictured below, (@zpacefromspace) who is the Vice-Chair of Students for the Exploration and Development of Space, USA (SEDS-USA), as well as an astronomy graduate student at the University of Wisconsin-Madison.

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