Dust Might Reveal the Presence of Habitable Planets?
NASA’s next great space telescope is expected to launch in 2027. The Nancy Grace Roman Space Telescope is a powerful wide-field infrared telescope with 100 times the field of view of Hubble. The Roman Telescope has a number of scientific goals, one of which is to compile an exoplanet census in order to address issues about habitability.
A new study demonstrates how the Roman Space Telescope can measure dust in distant solar systems in order to aid in the search for habitable planets.
Zodiacal dust, also known as interplanetary dust, is found throughout our Solar System. Collisions between asteroids, Kuiper-Belt Objects, and cometary activity are the primary sources of dust. It isn’t primordial dust from the Solar System’s infancy. The most of the early dust has gone.
Our Solar System’s zodiacal dust is contained in an area extending from the Sun to the asteroid belt between Mars and Jupiter. When observed from a distance, the dust scatters sunlight and is the second-brightest object in the sky after the Sun. When investigating exoplanets, however, the dust is an issue. When a planet in a distant solar system has its own interplanetary dust, the haze can conceal the planets 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 reveals the amount and type of activity that is happening. Comets, for example, produce a lot of zodiacal dust as they move through the inner Solar System. A system with a lot of exozodiacal dust indicates a lot of cometary activity, according to astronomers.
The dispersion of dust is also a clue. The pattern of distribution could indicate that planets are molding debris with their orbits.
However, we don’t know much about dust in other systems right now. It’s difficult to see what’s going on.
“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 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.”
The discovery of exozodiacal dust and everything we can learn from it has opened up a new vista in astronomy. Cold dust at large distances from their host stars has been imaged by astronomers, material that is further away from the star than Neptune is from the Sun. However, no one has yet created a detailed depiction of the warmer dust nearer to stars.
Observations with the Large Binocular Telescope Interferometer have given us a preliminary knowledge of the warm dust. The HOSTS survey (Hunt for Observable Signatures of Terrestrial Systems) was a first step toward a better understanding of exozodiacal dust. HOSTS measured the brightness and density of heated dust floating in the habitable zones of neighboring stars, and it was a first step toward comprehending exozodiacal dust. Its observations helped determine how powerful future telescopes need to be to see distant exoplanets despite the dust’s presence.
The strong coronagraph of the Roman Space Telescope isn’t the only thing the telescope has to offer. Some of the constraints that Hubble has will not apply to it. The Hubble Space Telescope is in low-Earth orbit, where it must contend with the presence of the Earth while studying distant objects. The Roman, on the other hand, will be at a lovely, stable location at 1.6 million kilometers (1 million miles) from Earth, at LaGrange Point 2 (L2).
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 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.