Current Limitations & Future Study
Ever since the first exoplanets were discovered in the 1990s, astronomers continue to push the limits of technology in order to detect ever-more Earth-like systems. But there is only so much they can push before the technology we have today truly limits what is possible to detect. This page discusses some of the limitations as well as current and future space-based telescopes in order to push those limits even more.
|Image shows the current and future theoretical limits of finding extrasolar planets in terms of their distance from their parent star vs. mass. Figure from Lawson, Unwin, & Beichman (2004).|
Current limitations in finding exoplanets are two-fold - technology limitations and observational baseline limitations.
In terms of technology, astronomers are limited by the resolving power of telescopes (in more way than one). The simplest way to think about this is with the direct imaging method for finding exoplanets - the smaller the telescope, the less astronomers can actually see the planet separated from its parent star. Another fairly simple way to think about this is with the astrometry method: If astronomers can't very precisely determine the positions of stars, then any tiny wobble produced by the tug of an unseen planet will not be visible.
The current technological limitation for the radial velocity technique is the resolving power of gratings used in spectroscopy. Astronomers have to be able to separate light at very high resolution in order to observe the very small Doppler shifts that a planet will induce on its parent star.
The second set of limits is based upon how long we have been searching for exoplanets with current technology. The way to think about this is with the question, "How long do you have to observe a planet's effect on a parent star - in terms of the planet's year - in order to confirm that it's really a planet?" The basic answer to that is, "At least 1 year." So if we are trying to find a Jupiter-like planet that orbits its parent star at the distance that Jupiter orbits the Sun, then we have to observe the star for at least 12 Earth years (1 Jupiter year). For a Saturn-like planet, it expands to 29 Earth years.
That brings the discussion to the complicated graph on the right. This shows the current (as of approximately 2005 when the figure was made for a NASA proposal study) predicted future theoretical limits of detecting exoplanets. Each filled dot represents a known exoplanet. Each blue circle with a letter in it represents 7 planets in our solar system (Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune). The green vertical band that covers Earth and part of Venus is what is known as the "Habitable Zone" - a region of our solar system where an Earth-like planet could support life.
All of the different lines on the graph represent limits for various observational techniques. Anything above any of the lines should be observable with the labeled technique, while anything below it is below the detection limits. In general, the first branch of a technique - when going from left to right - is a limit imposed due to technology. The second branch is imposed due to the time of observations.
As you can see from the graph, an Earth-like planet is still beyond our theoretical detection limits. It will take the proposed Kepler and SIM NASA missions to search for planets. These are discussed in summary below.
The following is a list of the missions that have finished, are currently in operation, or are planned to be lanuched to try to find exoplanets. A brief summary is displayed, but you can click on the name of the mission to be taken to a page detailing the mission.
COROT ~ 2007-present ~ COROT, the "COnvection ROtation and planetary Transits" mission is a space-based exoplanet finding mission funded by the French Space Agency (CNES) and the European Space Agency (ESA). It has two main goals, the first being to find planets of a few Earth masses, and the second to study asteroseismology (the measurement of oscillations in stars like those we observe on the Sun). It found its first planet, COROT-Exo-1b, on May 3, 2007, orbiting a Sun-like star 1500 light-years away.
Kepler ~ 2009-? ~ Kepler is a space-based photometer (a light-measuring device). It is currently scheduled to be launched on February 16, 2009. The original launch date has suffered several delays due to budget cuts. It will only use the transit method for finding exoplanets, but it will observe 100,000 stars simultaneously. Data from the mission will also be used to study variable stars and asteroseismology.
SIM ~ 2015?-? ~ SIM, the "Space Interferometry Mission," was originally supposed to be launched in 2005. Due to severe budget cuts, it has been delayed to "no earlier" than 2015 or 2016. Assuming it is actually built and launched, the craft is supposed to use optical interferometry to accomplish the goal of direct imaging of exoplanets. It has the additional goal of constructing a map of our Galaxy.