Ever since we learned the true extent and wonders of our own solar system, we have speculated on the existence of planets orbiting other stars.
Subscribe now for unlimited access.
or signup to continue reading
We call these planets extra-solar or exoplanets.
Finding exoplanets is challenging work but in only the last thirty years, we have discovered over 4000. As the distance between planets and their Sun is much smaller than their distance away from us, most exoplanets can't be resolved by our best telescopes. We therefore need to resort to indirect means of detection.
The most successful method has been the transit method which looks for very small changes in the light from a star as the planet moves in front of it. This method doesn't allow us to directly 'see' the planet. Instead, we are observing the light coming from the star for a continuous period. When a planet moves in front of the star, the star brightness will drop by a very small amount which can only be detected by sensitive instruments such as those on NASA's Kepler and TESS missions.
Another indirect method is known as radial velocity. As planets orbit their stars, they have a small gravitational influence. This causes the star to wobble back and forth over the course of an orbit. We can measure these changes in the star's velocity to infer the presence of a planet. This velocity can also help us determine the planet's mass.
Together, the transit and radial velocity methods have discovered over 95 per cent of all exoplanets. However, both methods are limited by the fact that we need to observe the system for at least a full orbit. For the transit method, we need at least three.
These methods are therefore biased toward planets with orbital periods of a few days or weeks; much shorter than the planets in our solar system.
Mercury, the closest planet to the Sun, has a period of three months while Jupiter, the largest planet, has an orbital period of twelve years.
Transit and radial velocity methods are unlikely to discover planets with these long orbital periods. In order to search for long period planets, we can try direct imaging in which we observe light from the planet itself. This doesn't require us to observe a full orbit. It is a challenging method due to the fact that stars are much brighter than their planets which are mostly reflecting light from the star.
However, if we observe a young star where planets may still be forming, it is possible to observe the light from these planets directly. As gas giants such as Jupiter form, they get hot and start to glow in infrared wavelengths. They then fade over time as they cool down.
It is, therefore, possible to see another solar system being born, which has been recently observed around the star PDS 70.
By observing these systems in early stages, we can determine where planets are likely to form and gain insight into how they form.
- Alex Wallace is a PhD student at Mount Stromlo working on the detection of young planets in other solar systems (known as exoplanets) through direct imaging.
