Gravitational Microlensing: The Hunt for Exoplanets

19 April 2016

Introduction


Dr Nicholas Rattenbury
Dr Nicholas Rattenbury

“Do other stars have planets?” This has been one of the most captivating questions in modern astronomy. And we now have an answer: yes, many stars do have planets of their own. This answer now opens up the dramatic possibility that some of them, or their moons, might harbour life.

A key tool for finding exoplanets is gravitational microlensing. When two stars line up in the sky, the closer star’s gravitational field becomes a giant lens, magnifying the light from the star behind it. As the stars line up more closely, the lensing effect increases. As they move out of alignment afterwards, it decreases. When you graph that, you get a characteristic “magnification curve”. If the closer star has planets, they can reveal themselves by enhancing the lensing effect. This creates a blip on the curve. In exoplanet research, astronomers trawl the night sky searching for these magnification events. 

University of Auckland scientists, led by Dr Nicholas Rattenbury, are persistent star-gazers playing a key role in this research. They’re locating alien solar systems as part of a major international research collaboration. This research helps us to understand how planets form. It also answers questions about how many other solar systems look like our own.

A diagrammatic representation of gravitational microlensing.
A diagrammatic representation of gravitational microlensing. By Phil Yock © University of Auckland

The details


The Micro-lensing Observations in Astrophysics (MOA) project

Astrophysicists at the University of Auckland are involved with MOA. MOA uses a dedicated 1.8-m telescope at Lake Tekapo, New Zealand. Its observations combine with others from around the world. Planets within a lens system may cause a brief deviation lasting several hours if they’re up to a few times the mass of Earth.

Microlensing highlights planets that orbit between 1.6 and 2.6 astronomical units from their star. (One AU is the distance between the Sun and Earth. It’s a common and useful unit of measurement in astronomy.) This region is not easily investigated by other techniques. And so microlensing complements findings from other research. 

Many eyes on the skies

Traditionally a wide-field telescope would observe many fields over each night. A more recent technique involves the observation of fewer fields but more frequently. New Zealand’s MOA-2 telescope has been conducting such surveys for some time. In 2014 the KMTNet system of three 2-m telescopes became fully operational, significantly increasing the worldwide planet discovery rate. 

Data crunching

The data from these new projects requires analysis. Exoplanet microlens modelling is an intricate mathematical challenge even at current volumes. With the numerous parameters involved, ensuring that an accurate model is found for a given set of data requires sophisticated algorithms and experience.

 

A graphical representation of the same event. The red line is an anomalous brightening then fade-back caused by a planet orbiting the lens star.
A graph of the same event. The red line is an anomalous brightening then fade-back caused by a planet orbiting the lens star. By Nicholas Rattenbury © University of Auckland
The Mt John observatory, Tekapo, where the MOA team conducts micro-lensing research
The Mt John observatory, Tekapo

Until now, data modellers have been able to apply their experience directly to analysis. As the rate of planetary events increases, this will become logistically impossible. The development of an automated system is an important aspect of Dr Rattenbury’s research. This system will need to use the most sophisticated algorithms reliably and efficiently within a distributed computing environment.