Dust Might Reveal the Presence of Habitable Planets

NASA’s next great space telescope should launch no later than 2027. The Nancy Grace Roman Space Telescope is a powerful wide-field infrared telescope that will create panoramic fields of view 100 times greater than Hubble’s. The Roman Telescope has a variety of scientific objectives, and one of its jobs is to complete a census of exoplanets to answer questions around habitability.

A new study shows how the Roman Space Telescope can measure the dust in distant solar systems to help find habitable planets.

Our Solar System contains a type of dust called zodiacal dust or interplanetary dust. The dust is primarily the result of collisions between asteroids, Kuiper-Belt Objects, and also from cometary activity. It’s not primordial dust from the early days of the Solar System. Most of that early dust is gone.

The zodiacal dust in our Solar System is contained in a region from near the Sun to the asteroid belt between Mars and Jupiter. The dust scatters sunlight and, when viewed from a distance, is the second-brightest thing in the sky next to the Sun. But the dust creates a problem when studying exoplanets. When a remote solar system has its own interplanetary dust, the haze can obscure the planets in that system in optical light.

While the dust can be a problem, it can also be a valuable signal.

Astronomers can use the Roman Telescope to search for this dust in distant solar systems. If astronomers find exozodiacal dust like this around a star, it indicates the presence of rocky bodies. That increases the likelihood of habitable planets. If the Roman doesn’t find this dust, that’s a win, too: the solar system is a good target for future observations with optical telescopes because there’s no dust in the way.

The new paper is titled “Sensitivity of the Roman Coronagraph Instrument to Exozodiacal Dust.” The lead author is Ewan Douglas, an assistant astronomy professor at the University of Tucson. The paper is published in Publications of the Astronomical Society of the Pacific.

“No one knows much about exozodiacal dust because it’s so close to its host star that it’s usually lost in the glare, making it notoriously difficult to observe.”

Study co-author Bertrand Mennesson, JPL.

“If we don’t find much of this dust around a particular star, that means future missions will be able to see potential planets relatively easily,” said Douglas. “But if we do find this kind of dust, we can study it and learn all kinds of interesting things about its sources, like comets and asteroids in these systems and the influence of unseen planets on its brightness and distribution. It’s a win-win for science!”

The amount of dust in a solar system indicates how much and what types of activity are going on. For example, a lot of zodiacal dust comes from comets when they pass through the inner Solar System. If astronomers find a system with lots of exozodiacal dust, it suggests a high level of cometary activity.

This image shows zodiacal light in the sky. It’s visible before sunrise or after sunset. Credit: Yuri Beletsky/ESO Paranal

The dust’s distribution is also a clue. The distribution pattern could signal that planets are sculpting the debris with their orbits.

But as it stands now, we don’t know much about the dust in other systems. It’s difficult to see.

“No one knows much about exozodiacal dust because it’s so close to its host star that it’s usually lost in the glare, making it notoriously difficult to observe,” said Bertrand Mennesson, Roman’s deputy project scientist at NASA’s Jet Propulsion Laboratory in Southern California and a co-author of the paper. “We’re not sure what Roman will find in these other planetary systems, but we’re excited to finally have an observatory that’s equipped to explore this aspect of their habitable zones.”

The telescope’s coronagraph plays a vital role in this. A coronagraph is like sunglasses. It blocks out the blinding glare of stars in distant solar systems, allowing astronomers to see fainter objects. Without a coronograph, it would be impossible to see the dust in other systems. Roman’s coronagraph will be the most powerful coronagraph ever used.

Other space telescopes, including the Hubble, have coronographs. But the Hubble’s is relatively simple compared to the Roman Telescope’s. The Roman Telescope’s coronagraph uses filters and deformable mirrors to subtract the powerful starlight from images, leaving light from planets, or zodiacal dust, behind.

“The Roman Coronagraph is equipped with special sensors and deformable mirrors that will actively measure and subtract starlight in real-time,” said paper co-author John Debes. Debes is an astronomer at the Space Telescope Science Institute in Baltimore. “This will help provide a very high level of contrast, a hundred times better than Hubble’s passive coronagraph offers, which we need to spot warm dust that orbits close to the host star.”

Exozodiacal dust and all we can learn from it is a new horizon in astronomy. Astronomers have imaged cold dust at great distances from their host stars, dust that’s further from the star than Neptune is from the Sun. But so far, nobody has imaged the warmer dust nearer to stars in any detail.

We have a preliminary understanding of the warm dust from observations with the Large Binocular Telescope Interferometer. The Hunt for Observable Signatures of Terrestrial Systems (HOSTS) survey was a stepping stone towards a greater understanding of exozodiacal dust. HOSTS determined the brightness and density of warm dust floating in nearby stars’ habitable zones, and it was a stepping stone towards understanding exozodiacal dust. Its observations helped determine how powerful future telescopes need to be to see distant exoplanets despite the dust’s presence.

The Roman Space Telescope has more to offer than its powerful coronagraph. It won’t suffer from some of the limitations that Hubble suffers from. The Hubble is in low-Earth orbit, where it has to contend with the Earth’s presence as it studies distant objects. But the Roman will be in a nice, stable location at the LaGrange Point 2 (L2), 1.6 million kilometres (1 million miles) from Earth.

Astronomers are eager to study the warmer exozodiacal dust closer to distant stars because there are critical differences between it and the more visible, colder dust further from the star. The dust near the star is dominated by rocky grains, whereas the more distant dust comprises ice grains. Different processes form the different grains, so one type isn’t an analogue for the other.

“By prospecting for this dust, we could learn about the processes that shape planetary systems while providing important information for future missions that aim to image habitable-zone planets,” co-author Debes said. “By finding out how much exozodiacal dust is in the way of possible planets in nearby systems, we can tell how large future telescopes will need to be to see through it. Observations from the Roman Coronagraph could offer a crucial steppingstone in the search for Earth analogs.”

The exozodiacal dust inhibits other aspects of exoplanet science, too. One way to determine potential habitability or detect biosignatures is by studying exoplanet atmospheres. But the dust can get in the way. “Since most reflected starlight and many biomarkers of interest fall at visible and NIR (near-infrared) wavelengths, the scattering of visible light by dust is the primary source of background flux that will limit detailed spectroscopic characterization of exoplanet atmospheres,” the paper states.

Hopefully, the Roman Space Telescope will pave the way for another mission called the Habitable Exoplanet Observatory (HabEx.) HabEx is still in the concept and design phase, and if it comes to fruition, its current proposed launch date isn’t until 2035. HabEx’s mission will be to image planetary systems around Sun-like stars directly. It’ll sense all types of planets, but the focus is on Earth-like worlds. HabEx will be able to image some of these planets directly. It’ll also analyze their atmospheres spectroscopically for habitability signatures or biosignatures.

The HabEx mission is unique in its approach. Rather than an onboard coronagraph, it’ll employ a separate starshade. The starshade would be about 77,000 km (48,000 miles) away from the telescope and would block the light from distant stars and allow light from planets to reach the telescope’s instruments.

In this study, the authors used simulations to determine how many of the pre-selected target stars for the HabEx mission would be observable by the Roman Telescope. The Roman Telescope’s advanced coronagraph gives it more power to observe planets in habitable zones than its predecessors. The idea is to gain some advance knowledge of the scattered background light from exozodiacal dust that HabEx will face when it begins its observations.

This graph from the study shows some of the predicted results from the Roman Telescope. It’s based on a sample of 149 target stars that are considered optimal targets for the HabEx mission. The graph shows that 74 of HabEx’s 149 target stars have at least a part of their HZ accessible to the Roman’s observations. Sixteen of them are observable down to their EEID, which means Earth-equivalent insolation distance, the distance from a star at which an exoplanet receives the same amount of solar energy as Earth. IWA means Inner Working Angle and is a geometrical limit in coronagraphs. The Roman’s IWA is a strength of its design. Image Credit: Douglas et al. 2022.

“This analysis has shown the Roman Coronagraph will place new limits on scattered light brightness from exozodiacal dust in the HZ of nearby stars,” the paper says. “Such a program would provide valuable insight into the scattered light background faced by future missions to image and spectrally characterize Earth-like planets.”

The detailed knowledge that the Roman Telescope will provide about exozodiacal light will help pave the way for the HabEx mission. “… foreknowledge of which systems have excess exozodiacal light will allow better optimization of future direct imaging searches for Earthlike planets.”

“This has the potential to optimize the exoplanet observing strategy of future missions by allowing selective targeting of less dusty systems…” the authors write.

The detailed measurements of exozodiacal dust will be just one of the Roman Telescope’s contributions to astronomy. And the link between the Roman and HabEx is only one inter-mission linkup. The Roman will also study dark energy and the expansion of the Universe, and in that regard, it’ll dovetail with the ESA’s Euclid mission.

We’re living in an excellent age for astronomy. The long-awaited James Webb Space Telescope will begin observations soon, the Nancy Grace Roman will launch in a few years, and the venerable Hubble Space Telescope seems almost invincible. Ground-based observatories like the Vera Rubin Observatory will come online soon, with super telescopes like the E-ELT and the Magellan Telescope joining it.

If there are Earth-like planets out there that host life, we may be on the cusp of finding some of them.

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Evan Gough

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