Stopping singles is already hard enough…

Exoplanet detection was front-line science not too long ago. However, we have now discovered over 5,000 stars and expect to find them around almost every star. The next step is to more fully characterize these planets to find those that can support life. Imaging them directly will be part of that effort.

But to do that, astronomers must block the light from the planet’s star. This is difficult in binary star systems.

When astronomers need to block out starlight to survey nearby planets, they use a telescopic attachment called a coronagraph. The Hubble Space Telescope has one, and so do many other telescopes. Very effective.

The Coronagraph effect is well established in single star systems. But what about binary star and multi-star systems? Binary stars are common in the Milky Way, and up to 85% of the stars in the Milky Way may be in binary systems. And there are many in our neighborhood. ESA’s Gaia spacecraft has discovered 1.3 million binary stars within 1,000 light-years of Earth.

You don’t have to look far to find multistar systems with exoplanets. Our closest 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 star slightly larger than Jupiter. Proxima Centauri is so faint that Alpha Centauri effectively resembles a binary star. Alpha Centauri A and B are also close together, and Proxima Centauri is in a much wider orbit around the main pair.

The Alpha Centauri system is an instructive example of the challenges faced by astronomers attempting to image exoplanets. Alpha Centauri A and B are about 40 astronomical units apart. The combined light of two sun-like stars so close together could easily drown out a much dimmer exoplanet. However, the new technology has several possibilities. It’s called Multi-Star Wavefront Control (MSWC).

The problem with blocking light from binaries is cross-contamination. Current coronagraphs can suppress light from a single star, but cannot manage cross-contamination from separate stars. Eliminating polluted light is critical for imaging exoplanets. That’s where MSWC comes in.

Multi-Star Wavefront Control is key to future missions. NASA hopes to launch the Nancy Grace Rome Space Telescope (NGRST) in 2027. It will be equipped with a technology demonstration coronagraph called CGI (CoronaGraphic Instrument) based on MSWC. Deformable Mirrors (DMs) are an important part of the system.

Deformable mirrors are not a new technology. Both the upcoming 30-meter telescope and the European Very Large Telescope use deformable mirrors. They are part of Adaptive Optics.

The DM system works for single stars or overlapping binary stars. But counteracting cross-contamination from non-overlapping binaries requires something else. It is the second part of the coronagraph of the Romans. ‘Super Nyquist wavefront control.’

The problem with binary systems is that the DM’s field of view (FoV) is limited. The DM can adapt to the light from a single star, but the binary companion is outside the FoV. The Nyquist system solves this problem by extending the FoV using hardware and software. The system essentially creates a grid of proxy stars for the binary’s secondary stars, and each proxy has a modified DM area. This creates a dark area outside the DM’s FoV. The advantage of this system is that it can be applied to any telescope with a deformable mirror. (A detailed explanation of how it works is here.)

Normally, space telescopes do not require adaptive optics. They are used in ground-based telescopes to offset the effect of the atmosphere on the telescope. The Nancy Grace Rome Space Telescope will be the first space telescope to use transforming mirrors. And if it goes well, a system based on NGRST’s will be part of NASA’s Habitable Worlds Observatory (HWO). HWO is the Habitable Exoplanet Observatory (HabEx) and the Large UV/Optical/IR Surveyor (LUVOIR).

However, you should thoroughly test your device before this happens. That’s happening at the Ames Coronagraph Experiment Laboratory and Subaru Telescope’s Subaru Coronagraph Extreme Adaptive Optics (SCExAO) instrument. The MWSC team is also testing on NASA’s Jet Propulsion Laboratory’s High Contrast Imaging Test Bed (HCIT).

The astronomy community knows that our search for exoplanets is hindered by starlight from binary star systems. We may miss a lot of them.

A 2021 paper examines the issue and concludes that not only do we fail to detect lost exoplanets in binary flashes, but we may also fail to detect what everyone wants to find: an Earth-like planet in the habitable zone.

The thesis is “TESS Observation of Exoplanet Host Star Spots: Understanding Binary Exoplanet Host Star Orbital Period Distribution”. It was published in The Astronomical Journal and lead author is Steve Howell of NASA Ames Research Center.

In their paper, the authors point out that they “established a binary rate of 46% in exoplanets host stars.” The team used the Gemini Observatory’s telescopes to study the stars hosting the planets discovered by TESS. They determined that we could easily miss the detection of an Earth-sized planet in a binary system. TESS relies on planets passing in front of its star to detect planets by dips in starlight. But the glare of other stars can easily hide the dip.

They examined hundreds of TESS stars and found that 73 of them were actually binary stars, details that TESS missed. Is Earth 2.0 or something close to it hidden somewhere around those stars? How many planets are obscured by the light of two stars?

“When you go out and look at the stars in the night sky, you might be seeing an Earth-like planet hidden in the starry glare,” said Ruslan Belikov, MSWC’s project manager. “Also, it is possible that the star you are looking at is a multi-star system. I can’t wait until we lift the veil on the starlight and unlock the mysteries of the planet.”

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