Telescopes

It’s Already Hard Enough to Block a Single Star’s Light to See its Planets. But Binary Stars? Yikes

Detecting exoplanets was frontier science not 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 characterize these planets more fully in hopes of finding ones that might support life. Directly imaging them will be part of that effort.

But to do that, astronomers need to block out the light from the planets’ stars. That’s challenging in binary star systems.

When astronomers need to block out starlight in order to examine a nearby planet, they use a telescopic attachment called a coronagraph. The Hubble Space Telescope has one, and so do many other telescopes. They’re very effective.

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

Coronagraph effectiveness is well-established in single-star systems. But what about binary stars and multiple-star systems? Binary stars are common in the Milky Way, and up to 85% of Milky Way stars may be in binary systems. And they’re plentiful in our neighbourhood, too. The ESA’s Gaia spacecraft found 1.3 million binary stars within 1,000 light-years of Earth.

We don’t have to look far to find a multi-star system with exoplanets. Our nearest stellar neighbour, the Alpha Centauri system, is a triple-star system. Alpha Centauri A and B are both bright, Sun-like stars. The system’s third star, Proxima Centauri, is a small red dwarf only slightly larger than Jupiter. Proxima Centauri is so dim that Alpha Centauri is effectively more like a binary star. Alpha Centauri A and B are also close to one another, 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 dim, far-flung companion.

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

The challenge in blocking out light from binary stars is cross-contamination. Current coronagraphs can suppress the light from a single star but can’t manage cross-contamination from a separate star. Eliminating the contaminating light is critical to imaging exoplanets. That’s where MSWC comes in.

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

Deformable mirrors aren’t brand-new technology. The upcoming Thirty Meter Telescope and European Extremely Large Telescope both utilize deformable mirrors. They’re part of Adaptive Optics.

The DM system works for single stars or for binary stars that overlap. But something else is needed to counteract cross-contamination from binary stars that don’t overlap. That’s the second part of the Roman’s coronagraph, and it’s called ‘super-Nyquist wavefront control.’

The problem in binary systems is that DMs have a limited field of view (FoV.) A DM can adapt to the light from 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 of 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 is here.)

This image helps explain how the system creates dark zones outside of the DM’s FoV. The DM grating diffracts an attenuated replica of star B into a sub-Nyquist region of star A. (The sun-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 blocks the light originating from star A. A side effect that can be seen on the diagram is the replica of A in the controllable region of B. This allows us to then search for planets around A in the box labelled DZ (Dark Zone.) Image Credit: Thomas et al. 2015.

Typically, adaptive optics aren’t needed on space telescopes. They’re 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 it goes well, a system based on the NGRST’s system will be 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 Surveyor (LUVOIR).

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

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

The astronomy community is aware that our search for exoplanets is hindered by the starlight in binary star systems. We could be missing a lot of them.

A 2021 paper examined the issue and concluded that not only are we failing to detect exoplanets lost in the glare of binary stars, but we might also be failing to detect what everybody hopes 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’s published in The Astronomical Journal, and the lead author is Steve Howell from NASA’s Ames Research Center.

In their paper, the authors point out that there’s an “established 46% binarity rate in exoplanet host stars.” The team used the telescopes at the Gemini Observatory to study planet-hosting stars found by TESS. They determined that we can easily miss detecting Earth-sized planets in binary systems. TESS relies on planets transiting in front of their star to detect them by the dip in starlight. But the glare of the other star can easily be hide the dip.

They examined hundreds of these TESS stars and found that 73 of them are really binary stars, a detail that TESS missed. Is Earth 2.0 or something close to it hidden somewhere around those stars? How many planets are we missing, drowned out by the light of two stars?

“Just imagine — when you go outside and look at a star in the night sky, you might be looking at a planet just like the Earth, hidden in the star’s glare,” said Ruslan Belikov, the project lead for MSWC. “Also, chances are that the star you’re looking at is a multi-star system. I just can’t wait until we lift veils of starlight to unlock the secrets that lie on the planets within.”

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    Evan Gough

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