Caption: "The population of exoplanets detected by the Kepler spacecraft (yellow dots) compared to those detected by other surveys using various exoplanet discovery methods: radial velocity (AKA Doppler spectroscopy: light blue dots), transit method (pink dots), direct imaging (green dots), gravitational microlensing (dark blue dots), and pulsar timing (red dots). For reference, the horizontal lines mark the radii of Jupiter, Neptune, and Earth, all of which are displayed on the right side of the diagram. The colored ovals denote different planet types: hot Jupiters (pink), cold gas giant (purple), ocean planets and ice giants (blue), rocky planets (yellow), and lava planets (green). The shaded gray triangle at the lower right marks the exoplanet frontier that will be explored by future exoplanet surveys. The Kepler spacecraft has discovered a remarkable quantity of exoplanets and significantly advanced the edge of the frontier." (Somewhat edited.)
The "Frontier" is the region of small planets at large mean orbital radii. Both prime exoplanet discovery methods (Doppler spectroscopy method and the transit method) are biased against these planets---that's why "Frontier" is the frontier---the region still to explore and discover.
For further reference, here is a semi-exhaustive list of planet types whose statuses range from certainly existant to just hypothetical:
-
Planet types:
carbon planet,
circumbinary planet,
Chthonian planet,
coreless planet,
eccentric Jupiter,
exomoon,
extragalactic planet,
gas giant,
Goldilocks planet,
helium planet,
hot Jupiter,
hot neptune,
iron planet,
lava planet,
ocean planet (AKA water world),
rocky planet,
rogue planet,
pulsar planet,
super-Earth.
For a more exhaustive list, see Wikipedia: List of planet types: List.
Credit/Permission:
NASA /
Ames Reseachr Center /
Natalie Batalha /
Wendy Stenzel,
2017
(uploaded to Wikipedia
by User:Drbogdan,
2017) /
Public domain.
Image link: Wikimedia Commons:
File:ExoplanetPopulations-20170616.png.
Local file: local link: exoplanet_populations.html.
File: Planetary systems file:
exoplanet_populations.html.