How Gravitational Microlensing helps spot new exoplanets

Gravitational microlensing is astronomers’ best method for discovering exoplanets far from Earth, but its latest application demonstrates that the technique can deliver an abundance of surprises.

A phenomenon known as gravitational microlensing — the curving of light due to the presence of mass — has been used to identify a new exoplanet orbiting a nearby star in the Taurus constellation.

The exoplanet has a mass slightly larger than that of Neptune and orbits its parent star — which is cooler than the Sun — at an orbital radius similar to that of the Earth’s. Around cooler stars, this region is generally restricted to the formation of gas-giants, but this finding suggests that Neptune-sized stars could be more common than previously believed at this orbital distance.

The exoplanet was first spotted by amateur astronomer Tadashi Kojima, Gunma Prefecture, Japan, and reported on November 1st, 2017. Follow-up observations were made by astronomers using telescopes across the Earth determined that the sighting was an example of gravitational microlensing.

Artist’s conception of the exoplanet and its host star. (The University of Tokyo)

This isn’t the first time that an exoplanet has been spotted using gravitational microlensing — but other observations using this technique have spotted stars lying closer to galactic centre, where star populations are dense.

This object was found in the opposite direction as viewed from Earth where stars become much sparser. Because microlensing events are short-lived and their dependence on objects of considerable mass they are more frequent were stars too are more common, making this finding somewhat unusual.

This exoplanet system represents both the closest and brightest as seen from Earth in the catalogue of exoplanet systems discovered thus far with the microlensing technique.

As such, it represents an ideal choice for follow-up investigations.

How does gravitational lensing help find exoplanets?

“Matter tells space how to curve.

Space tells matter how to move.”

-John Wheeler

Above is perhaps the most famous quote regarding one of the keys results from Einstein’s theory of General Relativity — the idea that the presence of mass causes a deformation of spacetime. It is this deformation that we describe as gravity — inaccurately described as a force by Newton.

This distortion of spacetime is often represented by imagining a large rubber sheet stretched out having increasingly heavy spherical objects placed upon it. A bowling ball creates a larger ‘dent’ than an apple or a tennis ball. If we then roll a marble past this indentation, its path will curve around the dent. The greater the indentation, the larger the diversion of the object’s path. Of course, this analogy only covers the distortion in 2 dimensions, whilst the actual distortion is 3 dimensional.

The deformation if spacetime by a massive body can be visualised as a massive object placed onto a stretched rubber sheet

One thing Wheeler missed from the above quote is that space also tells energy how to move. This means that as light travels in a straight line past an object of mass, its path is also distorted — thus the massive body can be described as acting as a lens of sorts. Again, just as with the marble in our analogy, the greater the mass, the greater the amount the path of that light is bent.

The gravitational lensing of a distant quasar by an intermediate body forms a double image seen by astronomers on Earth. (Lambourne. R, Relativity, Gravitation and Cosmology, Cambridge Press, 2010.)

This bending of light can make objects appear much further apart than they normally would, as well as making objects normally hidden behind the lens visible. In extreme cases, because the change in light path can affect the time at which they reach Earth, the effect can make the same star or galaxy appear at different points in the night sky — even forming structures called ‘Einstein rings’ all comprised of multiple images of the same object.

We call this effect gravitational lensing. Microlensing sees a star or smaller body fulfilling the role as the lens. This means the light is only bent slightly, hence the prefix ‘micro’.

The gravitational microlensing effect results from the bending of space-time near an object of given mass that is predicted by Einstein’s general theory of relativity. An object, such as a star, crossing our line of sight to a more distant source star will affect the light from that star just like a lens, producing two close images whose total brightness is enhanced. If the lensing star is accompanied by a planet, one can (potentially) observe not only the principal effect from the star, but also a secondary, smaller effect resulting from perturbation by the planet. ( Beaulieu et al)

When a star has a low-mass companion — such as a planet — in orbit around it close enough that it passes one light stream emerging from the star, the gravitational effect of that body bends that stream and creates a third image.

Astronomers on Earth see this event as a short-lived spike in brightness, lasting anywhere from a few hours to a few days. These spikes are therefore a great indicator of the presence of a planet around the source star. Some characteristics about the planet such as its mass and its orbital distance can be ascertained from the intensity and duration of the light curve spike.

THE MICROLENSING LIGHT CURVE OF PLANET OGLE–2005-BLG-390LBThe general curve shows the microlensing event peaking on July 31, 2005, and then diminishing. The disturbance around August 10 indicates the presence of a planet. (ESO)

In the land of the Gas Giants

Akihiko Fukui at the University of Tokyo and his team used the 188-cm telescope and 91-cm telescope at NAOJ’s Okayama Astrophysical Observatory and 11 other telescopes located at various locations around the world to observe this phenomenon for 76 days and collected enough data to determine the characteristics of the exoplanet system in question.

They were able to determine that the host star has a mass about half the mass of the Sun and that the exoplanet around it has an orbit similar in size to Earth’s orbit, with a mass about 20% greater than Neptune’s.

When the central star is cooler than the Sun, this region is generally considered more favourable for gas giants as it is the orbital distance at which water condenses into ice — conditions required by gas giant formation.

Currents estimates say the probability of finding a Neptune-like planet at such a distance around a cool star is just 35%. Thus, finding a planet like this by nothing but chance alone suggest that they may be more frequent in these orbits than currently theorised.

Despite its utilisation in the discovery of a nearby exoplanet, microlensing remains our best-known technique for the discovery of exoplanets at exceptionally large distances from earth. But these results show, as a technique for spotting exoplanets close-by, microlensing is capable of delivering some surprises.