Last year, using the exoplanets discovered by the Kepler space telescope as a guide, astronomers took a statistical stab at estimating the number of exoplanets that exist in our galaxy. They came up with at least 50 billion alien worlds. Today, astronomers from the Space Telescope Science Institute (STScI) in Baltimore, Md., and the PLANET (Probing Lensing Anomalies NETwork) collaboration have taken their own stab at the “galactic exoplanetary estimate” and think there are at least 100 billion worlds knocking around the Milky Way.
Why has the estimate doubled? The key difference here are the methods used to detect alien worlds orbiting distant stars. The Kepler space telescope watches the same patch of sky — containing around 100,000 stars — and waits for slight “dips” in starlight brightness. This dip occurs when an exoplanet passes in front of its parent star, thereby blocking a tiny fraction of light from view. This slight dimming effect is known as a “transit” and when four transits are detected by Kepler, the announcement of a confirmed exoplanet can be made.
The “transit method” has proven itself to be an excellent way of spotting exoplanets, but the method favors the detection of large exoplanets and exoplanets that orbit close to their stars. Pretty obvious really; the closer or the larger the exoplanet, the more starlight can be blocked and the bigger the “dip.” However, to arrive at their “galactic exoplanetary estimate” the PLANET team employed a rather different (and more random) exoplanet detection method known as “microlensing.” Microlensing depends on a lot of patience and a lot of luck, but given enough time and enough stars, exoplanets can be discovered this way.