Currently, 5,788 exoplanets have been confirmed in 4,326 star systems, while thousands more candidates await confirmation. So far, the vast majority of these planets have been gas giants (3,826) or Super-Earths (1,735), while only 210 have been “Earth-like” – meaning rocky planets similar in size and mass to Earth. What’s more, the majority of these planets have been discovered orbiting within M-type (red dwarf) star systems, while only a few have been found orbiting Sun-like stars. Nevertheless, no Earth-like planets orbiting within a Sun-like star’s habitable zone (HZ) have been discovered so far.
This is largely due to the limitations of existing observatories, which have been unable to resolve Earth-sized planets with longer orbital periods (200 to 500 days). This is where next-generation instruments like the ESA’s PLAnetary Transits and Oscillations of stars (PLATO) mission come into play. This mission, scheduled to launch in 2026, will spend four years surveying up to one million stars for signs of planetary transits caused by rocky exoplanets. In a recent study, an international team of scientists considered what PLATO would likely see based on what it would see if observing the Solar System itself.
The study was led by Andreas F. Krenn, a PhD student at the Space Research Institute at the Austrian Academy of Sciences. He was joined by researchers from the Observatoire Astronomique de l’Université de Genève, Aix Marseille University, the Columbia Astrophysics Laboratory, the Leibniz Institute for Astrophysics Potsdam (AIP), the Institute of Astronomy at KU Leuven, the National Center for Atmospheric Research, and the Kanzelhöhe Observatory for Solar and Environmental Research at the University of Graz. The paper that describes their research recently appeared in the journal Astronomy & Astrophysics.
As they note in their study, an Earth-like planet orbiting within the HZ of a G-type star would be a prime target to search for biosignatures. These include oxygen gas, carbon dioxide, methane, ammonia, and water vapor in the atmosphere, as well as indications of photosynthesis taking place on the surface – i.e., the vegetation red edge (VRE). This has been very difficult for telescopes as Earth-like planets are more likely to orbit closer to Sun-like stars, making it difficult to obtain data on their atmospheres using either Direct Imaging or transmission spectra.
This latter technique involves the Transit Photometry (or the Transit Method), where astronomers measure the light curve of distant stars for periodic dips in brightness. These are often the result of exoplanets passing in front of the star (i.e., transiting) relative to the observer. To date, the vast majority of exoplanets – more than 4,300, or 74.5% – have been confirmed using this method. When the conditions are right, astronomers sometimes observe light as it passes through the exoplanet’s atmosphere, which is then studied using spectrometers to determine its chemical composition.
But as Krenn told Universe Today via email, this has been a significant challenge for astronomers:
“The main difficulty is the small signals that such planets generate. For example, the radial velocity amplitude of the Earth is roughly 0.1 m/s. This is about the speed of a giant Galapagos tortoise. That means that if a distant observer would like to see the Sun’s motion around the common center of mass of the Earth-Sun system, they would need to see the Sun move at the speed of a giant Galapagos tortoise from light years away.
“Similarly, the relative amount emitted by the Sun that is blocked by the Earth when a distant observer observes the Earth transiting across the solar disk is 84 parts per million, which is 0.0084%. So a distant observer would need to see the light of that star being dimmed by 0.0084% in order to detect Earth.”
Moreover, Krenn added that existing spectrographs have not been precise enough to measure such small signals. Whereas exoplanet-hunting missions like the ESA’s CHaracterising ExOPlanets Satellite (CHEOPS) have managed to obtain spectra from transiting exoplanets, several transit events were needed to achieve this precision. This isn’t easy when dealing with planets like Earth with longer orbital periods that fit into the 200- to 500-day range. Lastly, instrumental effects and stellar variability can be orders of magnitude larger than a planetary signal.
This is expected to change considerably with the ESA’s next-generation PLAnetary Transits and Oscillations of stars (PLATO) space telescope. This mission will rely on a multi-telescope approach involving 26 cameras, including 24 “normal” cameras organized in 4 groups and 2 “fast” cameras for bright stars. These instruments will continuously observe the same area of the sky for at least two years to detect transit signals by Earth-like planets around solar analogs. Said Krenn:
“PLATO’s photometric instrument will be precise enough to detect the transit of an Earth-like planet orbiting a solar-like star using a single transit event. Supported by its stellar variability program and ground-based follow-up campaign, we will hopefully be able to correctly account for the influences of noise sources. In short, PLATO will utilize the interdisciplinary of exoplanet science on a whole new level. It will combine high-precision photometry, up-to-date data analysis tools, a dedicated stellar variability program, and its own ground-based follow-up campaign.
“Experts from all of these fields will work together to try and make the detection of these tiny planetary signals possible. Additionally, PLATO will also utilize a special observing strategy that allows it to observe thousands of stars a the same time and produce 2 years of almost continuous photometric data for each of them.”
To assess what PLATO might see when observing thousands of Sun-like stars for Earth analogs, the team modeled the impact of short-term solar variability using the Sun as a proxy. This consisted of using data obtained by the Helioseismic and Magnetic Imager (HMI) aboard NASA’s Solar Dynamics Observatory, which has been observing the Sun continuously since 2010. Using 88 consecutive days of HMI observations, they injected Earth-like transit signals and noise models into the data and simulated PLATO observations for five scenarios and five stellar magnitudes.
Their results showed that transit signals can be reliability detected with a high signal-to-noise ratio for bright targets, but still very likely for faint ones. They further found that the PLATO mission has a good shot at precisely and accurately measuring the size of Earth-like planets, one of its chief objectives. As Krenn explained, these findings could help inform the PLATO mission and assist in finding the signals of Earth analogs amid all the noise, though much work needs to be done to ensure all sources of noise are accounted for:
“In our analysis, we focused only on the effects of short-term variability, which we know is only one of many noise sources that will affect PLATO observations. We have seen that even correctly accounting for this single type of noise can be challenging. The final analysis of PLATO data will need to combine a variety of complex noise models simultaneously to correctly account for all of the different noise sources. I think our research has shown that we need to have an in-depth understanding of individual noise sources but, at the same time, also need to learn how to best combine all of the individual models.”
Other next-generation instruments, such as the James Webb Space Telescope (JWST), the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) telescope, and the Nancy Grace Roman Space Telescope will also allow for the discovery and characterization of countless exoplanets using the Direct Imaging Method. Along with upcoming ground-based observatories, these missions will rely on advanced optics, coronographs, and spectrometers to locate more Earth analogs and analyze their atmospheres and surfaces for evidence of life. Soon enough, astronomers will do away with terms like “potentially habitable” and be able to say with confidence that an exoplanet is “habitable” (and perhaps even “inhabited”!)
Further Reading: Astronomy & Astrophysics