It is arduous sufficient to dam the sunshine from a single star to see its planets. However double stars? whoops

The discovery of exoplanets was frontier science not so long ago. But now we’ve found over 5,000 of them, and we expect to find them around almost every star. The next step is to more fully characterize these planets in hopes of finding those that might support life. Your direct mapping will be part of this effort.

But to do that, astronomers have to block out the light from the planet’s stars. This is a challenge in binary star systems.

When astronomers need to block starlight to study a nearby planet, they use a telescope attachment called a coronagraph. The Hubble Space Telescope has one, and so do many other telescopes. They are very effective.

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This Hubble image shows the star AB Aurigae and the exoplanet AB Aurigae b. Hubble’s coronagraph (black circle) blocked the star’s light, making the exoplanet visible. The exoplanet is the bright spot under the coronagraph. The white star icon marks the position of AB Aurigae. Image Credits: NASA, ESA, T. Currie (Subaru Telescope, Eureka Scientific Inc.), A. Pagan (STScI); CC BY 4.0

Coronagraph effectiveness is well established in one star systems. But what about binary stars and multiple star systems? Binaries are common in the Milky Way, and up to 85% of the stars in the Milky Way may be in binary systems. They are also plentiful in our neighborhood. ESA’s Gaia spacecraft has found 1.3 million binary stars within 1,000 light years of Earth.

We don’t have to look far to find a multiple star system with exoplanets. Our nearest stellar neighbor, the Alpha Centauri system, is a triple star system. Alpha Centauri A and B are both bright, sun-like stars. The third star in the system, Proxima Centauri, is a small red dwarf only slightly larger than Jupiter. Proxima Centauri is so faint that Alpha Centauri effectively resembles more of a binary star. Alpha Centauri A and B are also close together, while Proxima Centauri is in a much wider orbit around the main pair.

This image shows how Alpha Centauri A and B appear as one bright star, while Proxima Centauri is a faint, distant companion.

The Alpha Centauri system is an instructive example of the challenge astronomers face when trying to image exoplanets. Alpha Centauri A and B are only about 40 astronomical units apart. The combined light from two Sun-like stars so close together can easily drown out their much fainter exoplanets. But a new technology shows promise. It’s called Multi-Star Wavefront Control (MSWC).

The challenge of blocking the light from binary stars is cross-contamination. Current coronagraphs can suppress the light from a single star, but cannot handle cross-contamination from a separate star. Eliminating the polluting light is crucial for imaging exoplanets. This is where MSWC comes in.

Multi-Star Wavefront Control is at the heart of an upcoming mission. NASA hopes to launch its Nancy Grace Roman Space Telescope (NGRST) in 2027. It will carry a technology demonstration coronagraph called CGI (CoronaGraphic Instrument) based on MSWC. Deformable Mirrors (DM) are a critical part of the system.

Deformable mirrors are not brand new technology. The upcoming Thirty Meter Telescope and the European Extremely Large Telescope both use deformable mirrors. They are part of the adaptive optics.

The DM system works for single stars or for double stars that overlap. But something else is needed to counteract cross-contamination from non-overlapping binaries. This is the second part of the Roman’s coronagraph and is called “Super Nyquist Wavefront Control”.

The problem in binary systems is that DMs have a limited field of view (FoV). A DM can adjust to the light of a single star, but a binary companion is outside the FoV. The Nyquist system gets around this by using hardware and software to expand the FoV. The system basically creates a grid of proxy stars for the secondary star in the binary and each proxy has a corrected DM region. This creates dark zones outside the DM’s FoV. The beauty of the system is that it can be adapted to any telescope with deformable mirrors. (A more detailed description of how it works can be found here.)

This image helps explain how the system creates dark zones outside the DM’s FoV. The DM grating diffracts an attenuated replica of star B into a sub-Nyquist region of star A. (The solar Nyquist region is the region where the deformable mirror coronagraph is effective.) The system treats the replica as another star. In this image, a coronagraph is blocking the light emanating from Star A. A side effect seen on the diagram is the replicating of A in the controllable region of B. This allows us to search for planets around A in the box labeled DZ (Dark Zone.) Photo credit: Thomas et al. 2015

Typically, adaptive optics are not required in space telescopes. They are used on ground-based telescopes to counteract the effect of the atmosphere on telescopes. The Nancy Grace Roman Space Telescope will be the first space telescope to use deformable mirrors. And if things go well, a system based on the NGRST system will become part of NASA’s Habitable Worlds Observatory (HWO). The HWO is a combination of two previous telescope ideas: the Habitable Exoplanet Observatory (HabEx) and the Large UV/Optical/IR Verifier (LUVOIR).

But before that can happen, the instrument must be thoroughly tested. This is done at the Ames Coronagraph Experiment Laboratory and at the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument on the Subaru Telescope. The team behind MWSC is also testing it at NASA’s Jet Propulsion Laboratory’s High Contrast Imaging Testbed (HCIT).

These images show MSWC being tested at NASA’s Jet Propulsion Laboratory’s High Contrast Imaging Testbed (HCIT). MSWC team members Eduardo Bendek, Ruslan Belikov, Dan Sirbu and David Marx are pictured left to right. Photo credit: NASA.

The astronomy community is aware that our search for exoplanets is hampered by starlight in binary star systems. We could miss many of them.

A 2021 paper examined the problem and concluded that not only do we not detect exoplanets lost in the glare of binary stars, but we may also not detect what everyone is hoping to find: Earth-like planets in habitable zones .

The paper is Speckle Observations of TESS Exoplanet Host Stars: Understanding the Binary Exoplanet Host Star Orbital Period Distribution. It was published in the Astronomical Journal and the lead author is Steve Howell of NASA’s Ames Research Center.

In their article, the authors point out that there is an “established binary rate of 46% among exoplanet host stars.” The team used the Gemini Observatory’s telescopes to study stars found by TESS that harbor planets. They found that it’s easy to miss spotting Earth-sized planets in binary systems. TESS relies on planets passing in front of her star to identify them by starlight bursts. But the other star’s glare is easily masked.

They examined hundreds of these TESS stars and found that 73 of them are actually binary stars, a detail that TESS missed. Is Earth 2.0 or something similar hidden somewhere around these stars? How many planets do we miss, drowned out by the light of two stars?

“Imagine – if you go outside and look at a star in the night sky, you might see a planet like Earth hidden in the star’s brilliance,” said Ruslan Belikov, project leader for MSWC. “Also, there’s a chance the star you’re looking at is a multiple star system. I can’t wait until we unravel veils of starlight to unravel the mysteries that lie upon the planets within.”

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