Finding Exoplanets using Microlensing
In 1992 scientists made a monumental discovery: the first detection of a planet beyond our Solar System. The new world was orbiting an exotic star called a pulsar, located over 2,000 light years away. A few years later, the planet 51 Pegasi B was discovered orbiting a main-sequence star, similar to our Sun. Today, thousands of extrasolar planets, better known as exoplanets, have been found.
The search for exoplanets is one of the fastest growing and most exciting fields in astronomy, that will perhaps one day answer the question of whether we are alone in the Universe.
Members of the LCO science team, in partnership with an international group of astronomers, are using LCO to detect and study exoplanets using three methods: Microlensing, Transit and Radial Velocity.
Microlensing
Most exoplanets can only be detected indirectly, because bright light from the stars they orbit drowns them out. One method of detection is microlensing: watching for a small blip in the light from a distant star, caused by the warping of space in the presence of a planet.
Microlensing can be used to find planets which are smaller and further from their star than any other detection method currently available. LCO successfully employs this technique to detect and study planetary systems. The network is able to undertake extended monitoring of targets around the clock, providing scientists with the ability to catch even the smallest planetary deviations.
The distant star's light gets bent by the closer star passing in front. This is gravitational lensing, resulting in multiple versions of the distant star being seen. Depending on the arrangement of stars, two or more versions of the distant star can be seen due to the lensing effect.
When a planet is orbiting the closer star, this can enhance the lensing effect briefly, creating a small blip in brightness. This is how we can use gravitational microlensing to detect exoplanets.
Transit Method
When a planet passes directly between an observer and its parent star, it blocks some of that star's light. For the brief period of time, the star becomes slightly dimmer. It's a tiny change, but if the dimming is detected at regular intervals and lasts a fixed length of time, it's enough to alert astronomers to the presence of an exoplanet.
This is the transit method. It is currently the most effective and sensitive method for detecting exoplanets. The transit method can provide information about a planet’s size, orbital period, temperature and the composition of its atmosphere.
However, a planet’s transit lasts just a tiny fraction of its total orbital period. As a result, it is extremely difficult to observe a transit in progress. The geographical distribution of LCO makes it possible to observe highly time-limited transit events whenever and wherever, which is a huge advantage for the field of transit follow-up.
LCO has made a huge impact in the field of exoplanets in the last few years by providing follow-up for transit surveys, such as the KELT (Kilodegree Extremely Little Telescope) survey which has discovered a number of planets around bright stars. These are of special interest because the intense starlight makes it possible to study the target planets in great detail.
Radial Velocity
Another method detects distant planets by measuring the movement of a star in response to the gravitational tug of a planet in orbit. If the planet is aligned edge-on to the Earth, we can observe this as a wobble in the star's light spectrum. As the star moves towards us, its spectrum appears shifted towards the blue (blueshift); when it is moving away, it is shifted towards the red (redshift). This is known as a 'Doppler' shift.
The gravitational pull from the planet is miniscule, so very accurate spectroscopic measurements are required, to detect this slight change in the hue of the spectrum. The success of this method was made possible by the development in recent years of extremely sensitive spectrographs, such a LCO’s Network of Robotic Echelle Spectrographs (NRES). NRES can detect even very slight movements of a star moving as slow as 3 meters per second.
In addition, the high-throughput and detailed spectra collected by NRES make it a game-changer, providing the LCO with the capability to observe many more targets than the current single-telescope facilities can.
This image is based on the Radial Velocity Method diagram by Jessica Barton, LCO