In 1995, astronomers reported the first detection of a planet orbiting another star.
The discovery of the planet 51 Pegasi b using the radial velocity method
would usher in a new era of astronomy.
The discovery helped pave the way for the HARPS (High Accuracy Radial Velocity Planet Searcher)
echelle spectrograph, which saw first light in 2003. HARPS has detected over 130 exoplanets, making
it the second most prolific exoplanet finder to date.
Much of our knowledge of exoplanets has come from the Kepler spacecraft, which was launched
into heliocentric orbit in 2009. Kepler's original mission was to continuously monitor the brightness of 145,000
stars, looking for dips in a star's brightness caused by transiting exoplanets. So far, it has detected over
1000 exoplanets, along with several thousand more candidates that await further confirmation.
Prior to Kepler, most discovered exoplanets were "Hot Jupiters" -- gas giants orbiting very close to their host star.
The most significant find with Kepler has been the abundance of "Super-Earths" -- rocky planets with masses between
Earth and Neptune.
The future of exoplanet astronomy appears bright. As NASA looks to the 2020 Decadal Survey to decide on the next major space-based
telescope, two of the four proposals (HabEx and LUVOIR) would have exoplanet imaging
as one of the core scientific goals.
HabEx (Habitable Exoplanet Observatory) would be dedicated to directly
imaging and studying the atmospheres of exoplanets via spectroscopy, prioritizing earth-like ones.
Similar capabilties would exist with LUVOIR (Large UV/Optical/Infrared Surveyor),
a swiss-army knife whose contributions would extend over multiple astronomical disciplines,
making it a true successor to the Hubble Space Telescope.
Earthshine refers to the sunlight reflected from Earth's atmosphere.
By studying the atmospheres of Exoplanets we can determine what elements are present,
and make inferences about the exoplanet, such as the presence of water through absorption
features in its spectrum.
The Trappist-1 System
40 light years from Earth lies the Trappist-1 system.
The star, Trappist-1a, is an ultra-cool red dwarf
with a surface temperature around 2550 K.
Its luminosity is only 5% of the Sun's and emits mostly
in the infrared.
Astronomers, using a 0.6 meter telescope at the La Silla
Observatory in Chile, discovered 3 earth-like planets
orbiting the star in 2015. Follow-up studies would bring the total to 7,
three of which are thought to be within the Habitable Zone.
Trappist-1a Light Curve
Shown below are some light curves of Trappist-1a. The transiting
exoplanet is indicated by the color of the diamond. Use these
plots to determine the radius of each exoplanet relative to the
host star.
If you click the image a new tab will appear, allowing you to zoom
in on the plot.
Are We Alone?
The search for earth-like planets inexorably brings with it the question of whether or not we are alone in the Universe.
The Italian philosopher Giordano Bruno (1548-1600) is perhaps
most famously associated with speculating that life exists on planets in other solar systems, although the idea was
championed as far back as the Greek philosopher Anaximander (610 - 546 BC).
To date, SETI (The Search for Extraterrestrial Intelligence) has been the most extensive undertaking
to search for life outside the solar system, but has yet to achieve success.
The most recent telescope designed specifically to search for intelligent life is the FAST
(Five hundred meter Aperture Synthesis Telescope) telescope in China, which achieved first light on 25 September 2016.
Further progress could be made in the coming years with the Square Kilometre Array (SKA),
which would be capable of picking up airport radars that are tens of light years away.
Of course, if we are capable of detecting such advanced civilizations, then they could also be listening in on us -- perhaps
hearing broadcasts from World War II or the Cold War.
Two possibilities exist: either we are alone in the Universe or we are not.
Both are equally terrifying. --Arthur C. Clark
