One proposal offers a unique method to directly image ExoEarths, or rocky worlds around nearby stars.
It’s the holy grail of modern exoplanet astronomy. As of writing this, the count of known worlds beyond the solar system stands at 6,520. Most of these are ‘hot Jupiters,’ large worlds in tight orbits around their host star. But what we’d really like to get a look at are ‘ExoEarths,’ rocky worlds (hopefully) like our own.
Now, a recent study out of the University of Paris, the European Southern Observatory (ESO) and the University of Cambridge entitled Exoplanets in Reflected Starlight with Dual-Field Interferometry: A Case For Shorter Wavelengths and a Fifth Unit Telescope at VLTI/Paranal suggests a method to do just that in the coming decade. This would involve one the most massive telescope complexes ever built: the Very Large Telescope. Based at Paranal Observatory in Chile, this array consists of four 8.2-metre telescopes working in concert via a method known as interferometry. The study advocates adding a fifth telescope, giving the VLT the capacity to see Jupiter-sized worlds shining directly in the host star’s light… and with a few key upgrades, the new and improved VLT could perhaps image ‘ExoEarths’ directly.
Pioneering Dual-Field Interferometry
Interferometry is the method of using superimposed waves collected from two telescopes to merge a signal into one image. This method allows for a resolution equivalent to the baseline between the two collecting instruments, bypassing the need for one enormous telescope. Long baseline radio interferometry can span continents, and there are plans to move the technique into space. Interferometry at visual wavelengths is a tougher proposition, one that’s just reaching its true potential.
Dual Field Interferometry uses the technique to simultaneously focus on two narrow fields in context within a larger field. One field is centered on the host star, and one on the target exoplanet. This can then minimize (subtract) photon shot noise from the primary, allowing for a clear view of the target world.
“With this technique, at the VLTI, we have a resolution equivalent to having a telescope of 130 meters,” lead author on the study Sylvestre Lacour (University of Paris) told Universe Today. “This allows us to distinguish the exoplanet’s light from the contamination by the stellar light, allowing to detect exoplanets very close to the star.”
“The term ‘dual’ in dual interferometry comes from the fact the we are observing at the same time the exoplanet and the star with the optical interferometer,” says Lacour. “This is necessary to be able to probe at the same time the phase of the stellar light and the phase of the exoplanet light, to be able to distinguish the two. By ‘phase’ I mean the phase of the electric field entering the interferometer.”
The Hunt for ExoEarths
The method is already being applied to reveal nearby worlds. “We typically observe exoplanets at a few tens of parsecs,” says Lacour. “They are massive exoplanets, more massive than Jupiter (between 4 and 10 Jupiter masses), and they are young, less than 50 million years (old). You can look for the results for the GRAVITY collaboration, operating the GRAVITY instrument at Paranal.”
One key technique used to overcome the effects of ‘shot noise’ is what’s termed as ‘apodization’. “Apodization is a way to decrease the contamination of the stellar light entering into our interferometer,” says Lacour. “It is similar to adding a coronagraph.”
Apodization makes ground-based systems such as the VLTI viable in terms of exoplanet science and direct detection. Other efforts such as the European Space Agency’s Proba-3 space telescope launching later in 2024 will use a free flying coronagraph to directly image exoplanets.
A pro to this method is it can characterize orbits within a few Astronomical Units from their host star. Other techniques observe planets very close in, or very far out. The downside of the method is that it’s a very difficult technique, right on the grim edge of what’s currently possible with existing telescopes.
The Future of Exoplanet Astronomy
There’s already a good case for plans to extend the VLTI baseline to a fifth instrument. This includes direct imaging for worlds known orbiting around nearby stars to include Proxima Centauri B and Tau Ceti e. Lessons learned from the VLTI could also work for the Extremely Large Telescope, which may see first light in 2028.
It’ll be exciting to see more nearby worlds revealed by this technique in the coming decade.