It is already difficult to block out the light of a single star to see its planets. But binary stars? Yikes
The discovery of exoplanets was groundbreaking science not so long ago. But now we’ve found more than 5,000 of them, and we expect to find them around almost every star. The next step is to fully describe these planets in hopes of finding planets that might support life. Filming them live will be part of that effort.
But to do this, astronomers need to block the light from the stars of the planets. This is a challenge in binary star systems.
When astronomers need to block starlight to examine a nearby planet, they use a telescopic facility called Coronagraph. The Hubble Space Telescope owns one, as do many others. It is very effective.
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 85% of the Milky Way Stars may be in binary systems. It is abundant in our region as well. The European Space Agency’s (ESA) 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 multi-star system 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 of the system, Proxima Centauri, is a small red dwarf slightly larger than Jupiter. Proxima Centauri is so dim that Alpha Centauri effectively looks like a binary star. Alpha Centauri A and B are also close together, while Proxima Centauri is in a wider orbit around the main pair.
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 40 AU away. The combined light of two close Sun-like stars could easily drown out their darker outer planets. But new technology holds some promise. It’s called Multi-Star Wavefront Control (MSWC.)
The challenge in blocking light from binary stars is cross-contamination. The current coronal vertebrae can block light from a single star but cannot manage cross-contamination from a separate star. Getting rid of polluted light is critical to imaging exoplanets. This is where MSWC comes in.
Multi-Star Wavefront Control is at the heart of an upcoming mission. NASA hopes to launch Nancy Grace Romanian Space Telescope (NGRST) in 2027. It will carry a technical presentation segment called CGI (CoronaGraphic Tool) based on MSWC. Deformable mirrors (DM) are an important part of the system.
Deformable mirrors aren’t an entirely new technology. The upcoming Thirty Meter Telescope and the European Very Large Telescope both use deformable mirrors. They are part of Adaptive optics.
The DM works with single stars or nested binary stars. But something else is needed to counteract cross-contamination from binary stars that don’t overlap. This is the second part of the Roman coronagraph, and is called “Nyquist Superwave Front Control.”
The problem with binary systems is that DMs have a limited field of view (FoV.) A DM can adjust to light from one star, but a binary companion is out of the FoV. Nyquist’s system gets around this by using hardware and software to extend the FoV. The system basically creates a network of proxy stars for the secondary star in the binary, and each proxy has a patched DM area. This creates dark areas 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 here.)
Usually, adaptive optics are not necessary in space telescopes. They are used on ground-based telescopes to counteract the effect of the atmosphere on telescopes. The Nancy Grace Romanian Space Telescope will be the first space telescope to use deformable mirrors. And if all goes well, the NGRST-based system will be part of the NASA system Habitable Worlds Observatory (HWO.) HWO is a combination of two previous ideas for telescopes: Habitable Exoplanet Observatory (HabEx) f Great UV/Optical/IR Surveyor (Louvre).
But before any of that happens, the tool needs to be thoroughly tested. This is happening at the Ames Coronagraph Experiment Laboratory and on the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument on the Subaru Telescope. The MWSC team is also testing it in the High Contrast Imaging Test (HCIT) at NASA’s Jet Propulsion Laboratory.
The astronomy community understands that our search for exoplanets is hampered by starlight in binary star systems. We may miss a lot of them.
A 2021 paper examined the problem and concluded that not only are we failing to detect exoplanets lost in the glow of binary stars, but we may also be failing to discover what everyone is hoping to find: Earth-like planets in habitable zones.
the paper “TESS exoplanet host star-spot observations: understanding the orbital period distribution of binary exoplanet hosts. Published in The Astronomical Journal, the lead author is Steve Howell of NASA’s Ames Research Center.
In their paper, the authors note that there is a “consistent binary binary rate of 46% in exoplanet host stars.” The team used telescopes at the Gemini Observatory to study the host stars of the planets found by TESS. They determined that we could easily miss detecting Earth-sized planets in binary systems. TESS relies on planets transiting in front of their star to detect them by dipping in starlight. But the glow of another star is easy to hide.
They examined hundreds of these TESS stars and found that 73 of them are actually binary stars, a detail TESS missed. Is Earth 2.0 or something close to it hidden somewhere around those stars? How many planets have we lost, drowned out by the light of two stars?
“Just imagine — when you go out and look at a star in the night sky, you might be looking at a planet just like Earth, hidden in the star’s glow,” said Ruslan Belikov, MSWC project leader. “Also, it’s possible that the star you’re looking at is a multiple star system. I can’t wait for us to lift the veil of the stars to reveal the secrets that lie on the planets within us.”
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