Searching for Alien Worlds

Methods of Searching for Alien Planets Pulsar Timing Astrometry Radial Velocity Transits Lensing Imaging

Planets cause stars to wobble. There are 3 ways to detect the wobbling of stars caused by planets: pulsar timing, astrometry, and radial velocity. Wobbling Stars

Pulsar Timing Some neutron stars beam light along their magnetic poles.

Pulsar Timing If the “searchlight” points towards earth, we see a pulsar. The timing of a pulsar’s light is normally extremely regular and precise. But if a planet orbits the pulsar, the pulsar wobbles back and forth, which causes the time between pulses to change. So we can search for planets around pulsars by checking whether their pulses occur at irregular intervals. Pulsar timing demo

In 1994, Penn State Prof. Alex Wolzczan discovered the first planet outside the solar system through timing of a pulsar. This pulsar has 3 small planets with masses of 0.02, 4.3, and 3.9 M  (M  =mass of Earth). These planets probably formed after the supernova explosion from the debris that was left behind. Pulsar Timing

In addition to pulsar timing, there are 2 other methods of detecting planets through the wobbles they induce in their stars. We can see the star physically wobbling on the sky (astrometry), or we can detect the star’s wobble via the Doppler effect as it moves towards us and away from us during the wobbling (radial velocity). Astrometry & Radial Velocity astrometry radial velocity

Astrometry If one observed the Sun from a distance of 30 light years, the size of its wobble on the sky (due mostly to Jupiter and Saturn) would be less than . In comparison, because of blurring by the atmosphere, stars have sizes of about 1  on the sky. It is very difficult to measure such small wobbles from the ground, so no planets have been discovered with astrometry, but future space telescopes should be more successful. Astrometry demo

Radial Velocity As the star wobbles back and forth due to the orbit of an unseen planet, its velocity relative to us changes slightly. So planets can be detected by measuring the change in a star’s velocity over time with the Doppler effect. Doppler shift demo

If you plot the velocity of a star over a long period of time, and find that the velocity changes regularly, you’ve found a planet. Radial Velocity

The first planet around a Sun-like star (51 Peg) was discovered via radial velocity monitoring in The data imply the presence of a planet with - a roughly circular orbit - a distance of A.U. - a mass of 0.46 M Jup It’s like a very hot Jupiter! Radial Velocity

Radial velocity has been very successful, discovering roughly 500 planets so far. this line corresponds to a given size of wobble in the star Hard to detect planets that have small masses or large orbits Mass

Transits If a planet’s orbit happens to be perfectly edge-on from our point of view, it will pass directly between us and its star. When this happens, the light from the star will decrease very slightly (less than 1%). Therefore, planets can be found by watching stars to see if they dim periodically in this way. Since stars are so far away, they appear as only points of light. As a result, we cannot see the planet moving across the face of the star as shown in this movie. We only detect the dimming of the total light from the star.

Ground-based telescopes have discovered roughly 100 planets using the transit method. Between 2009 and 2013, NASA’s Kepler satellite discovered 4000 candidate planets, 900 of which have been confirmed as planets so far.

Kepler searched for transiting planets by taking continuous images of a large area in the constellation of Cygnus. Because it is in space, it can detect much smaller dips in the light of the stars, and hence much smaller planets (even as small as Earth).

Searching for Earth-like planets Larger planets produce faster wobbles in the stars that they orbit, and they produce larger dips in the star’s light if they transit, making them easier to detect. Most of the planets discovered outside the solar system are gas giants that are unlikely to harbor life However, planets as small as Earth have been found (particularly with Kepler). Several of these planets appear to be near the habitable zones of their stars, so they could have liquid water on their surfaces.

Gravitational Lensing If a star/planet moves exactly in front of a background star, the brightness of the background star can be greatly magnified by the gravitational lens effect.

Gravitational Lensing In principle, the gravitational lens technique can detect planets of any mass. However, once the event is over, the planet is lost (since we are only seeing the background source). It is impossible to learn anything more about the planet. This method has discovered about 20 planets so far.

Detecting planets via gravitational lensing doesn’t require large telescopes since it’s just a matter of watching bright stars to see if their brightness changes. In fact, amateur astronomers with very small telescopes have played a large role in detecting planets with this technique. Gravitational Lensing

It’s very difficult to directly photograph planets because of the glare from the stars they orbit. Only a few planets that orbit very far from their stars have been detected in images, and this can only be done with the sharpest possible images (e.g., Hubble). Imaging 3 Jupiter masses 120 AU