The Purpose of HPF

The study of exoplanets has advanced rapidly in recent years.  We now know that planets form as an inevitable consequence of star formation, and that most stars in our Galaxy host planetary systems.  But to answer the so-called “Big Question”–that is, whether or not there exists life outside our own planet–we need more detail.  While the Kepler spacecraft has found thousands of new exoplanets, most of them lie too far away for us to characterize them in any detail.  In order to properly assess whether a planet is truly Earth-like, or capable of supporting life, we must find small, terrestrial planets around the closest stars to the Sun.  Then, we may begin to study their atmospheres, surface features, and potentially signs of life.

The Habitable Zone Planet Finder Spectrograph, or HPF, is designed to find those nearby small exoplanets.  Most of the stars closest to the Sun (the Solar neighborhood) are small, cool stars called M dwarfs.  M dwarfs emit most of their light in the infrared, or wavelengths redder than visible light, so HPF is targeting the wavelength range where these stars are the brightest.  When completed, HPF will survey several hundred nearby M stars, searching for the tiny changes in the stars’ motion created by the gravitational pull of low-mass planets.  At the end of the survey, we expect to find potentially habitable exoplanets that will be ideal for follow-up characterization by ground- and space-based telescopes currently in development.

Why M stars?

In addition to accounting for the majority of the Sun’s closest neighbors, the sizes and temperatures of M dwarf stars offer some important advantages for discovering habitable (or inhabited!) planets.  First, the lower masses of the stars means the gravitational tug of an Earth-mass planet will result in a larger stellar motion, resulting in a bigger Doppler signal than observed for Sunlike stars.

Here is the stellar “wobble” from an Earth-mass planet around the Sun (greatly exaggerated):

And here is the motion induced by the same planet around an M dwarf:

Because M dwarfs are also cooler, the habitable zone where liquid water can exist is much closer to the star.  According to Kepler’s third law of planetary motion, the shorter orbital radii of habitable planets around M stars results in shorter orbital periods, allowing us to discover them using less telescope time.

The shorter orbits of habitable-zone planets around M stars also increases the probability the planets will transit–or eclipse–their parent stars.  Transiting planets offer the best possible opportunity to characterize their atmospheres, as the apparent radius of a planet at various wavelengths can hint at its atmospheric composition.  Again, the smaller radii of M stars present an advantage, as the percentage of light blocked by a transiting planet (that is, the magnitude of the transit signal) is proportional to the squared planet-to-star radius ratio.  It is possible that if a habitable planet transits a nearby M dwarf, a space telescope like JWST could infer the presence of surface life based on telltale atmospheric signatures!


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