We present a novel, model-independent framework for studying the architecture of an exoplanetary system at the system level. This framework allows us to characterise, quantify, and classify the architecture of an individual planetary system. Our aim in this endeavour is to generate a systematic method to study the arrangement and distribution of various planetary quantities within a single planetary system. We propose that the space of planetary system architectures be partitioned into four classes: similar, mixed, anti-ordered, and ordered. We applied our framework to observed and synthetic multi-planetary systems, thereby studying their architectures of mass, radius, density, core mass, and the core water mass fraction. We explored the relationships between a system’s (mass) architecture and other properties. Our work suggests that: (a) similar architectures are the most common outcome of planet formation; (b) internal structure and composition of planets shows a strong link with their system architecture; (c) most systems inherit their mass architecture from their core mass architecture; (d) most planets that started inside the ice line and formed in-situ are found in systems with a similar architecture; and (e) most anti-ordered systems are expected to be rich in wet planets, while most observed mass ordered systems are expected to have many dry planets. We find, in good agreement with theory, that observations are generally biased towards the discovery of systems whose density architectures are similar, mixed, or anti-ordered. This study probes novel questions and new parameter spaces for understanding theory and observations. Future studies may utilise our framework to not only constrain the knowledge of individual planets, but also the multi-faceted architecture of an entire planetary system. We also speculate on the role of system architectures in hosting habitable worlds.
Citation: L. Mishra, Y. Alibert, S. Udry, C. Mordasini, A framework for the architecture of exoplanetary systems. I. Four classes of planetary system architecture, Astronomy and Astrophysics, Accepted December 2022 https://doi.org/10.1126/sciadv.abn2103
“What new discoveries from the James Webb space telescope (JWST) can I tell my nine-year old about?”
-The one question put to Google’s Bard AI program, which was designed to rival Microsoft’s ChatGPT. In response, Bard stated the JWST took the very first pictures of an exoplanet, which was incorrect. The first image of an exoplanet can be seen above, taken by the European Southern Observatory’s Very Large Telescope back in 2004. The exoplanet, called 2M1207 b, is a gas giant about five times the mass of Jupiter. Following the erroneous feedback, the stock of Google’s parent Alphabet dropped about $100 billion in value. Below is an image from another exoplanet spotted by the JWST last year, called HIP 65426 b, which is about six to eight times the mass of Jupiter.
Jupiter was already the king of the solar system, and new discoveries give the massive planet another way to reign supreme: It now has the most moons. Twelve new moons discovered orbiting Jupiter have been confirmed, bumping the count from 80 to 92, and knocking Saturn — which has 83 moons — down a peg.
In 2015, when NASA’s New Horizons spacecraft encountered the Pluto-Charon system, the Southwest Research Institute-led science team discovered interesting, geologically active objects instead of the inert icy orbs previously envisioned. An SwRI scientist has revisited the data to explore the source of cryovolcanic flows and an obvious belt of fractures on Pluto’s large moon Charon. These new models suggest that when the moon’s internal ocean froze, it may have formed the deep, elongated depressions along its girth but was less likely to lead to cryovolcanoes erupting with ice, water and other materials in its northern hemisphere.
…perhaps the biggest question of all — that of “Are we alone in the Universe?” — remains a mystery. While the current generation of ground-based and space-based telescopes can take us far into the Universe, this is a question that’s currently beyond our reach. To get there, we’ll need to directly image Earth-like exoplanets: planets with sizes and temperatures similar to Earth, but that orbit Sun-like stars, not the more common red dwarf stars like Proxima Centauri or TRAPPIST-1. Those capabilities are precisely what NASA is aiming for with its newly announced flagship mission: the Habitable Worlds Observatory. It’s an ambitious project but one that’s well worth it. After all, finding out we’re not alone in the Universe would quite possibly be the biggest revolution in all of science history.
I recommend checking out Alan Alda’s interview with astronomer Sara Seager in a recent Clear + Vivid podcast episode. MIT Professor Seager has focused her work on exoplanet atmospheres as well as another planet nearby – Venus. In the interview, she discusses her early work as well as her theories about the existence of life in the atmosphere of Venus. She also discusses her involvement with MIT’s Transiting Exoplanet Survey Telescope (TESS).
And while I do not remember it coming up during the interview, Professor Seager is also known for the Seager equation (shown below), which is less demanding than the Drake equation and focuses on any form of life on another planet (without reference to technology).
N = the number of planets with detectable signs of life
N* = the number of stars observed
FQ = the fraction of stars that are quiet
FHZ = the fraction of stars with rocky planets in the habitable zone
FO = the fraction of stars with observable planets
FL = the fraction of planets that have life
FS = the fraction of life forms that produce planetary atmospheres with one or more detectable signature gases
But in addition to the science, it was a fascinating discussion about Professor Seager’s life covering the early death of her husband from cancer, her attempts to get her life back on track, and her discovery later in life that she has autism. Most science stories focus on the work, but Mr. Alda has a unique way of drawing out the person in these interviews. It is a great episode, and you can read more about Professor Seager’s life and work in her book The Smallest Lights in the Universe.
The search for another Earth continues, and a professor from Michigan State may have found a perfect candidate. Joey Rodriguez, an assistant professor in MSU’s Department of Physics and Astronomy, was working with researchers involved with NASA’s Transiting Exoplanet Survey Satellite (TESS) when the discovery was made. Since being launched in 2018, the TESS spacecraft is surveying 200,000 stars looking for exoplanets that transit in front of their parent star and thereby periodically block part of the star’s light.
Back in 2020, the TESS team spotted the 100 light-year distant solar system and three exoplanets, but the latest finding includes Earth-size TOI-700e, which orbits in the habitable zone around parent star TOI-700. Its orbit is closer to that of Venus than Earth, yet still within the habitable zone.
In the Lansing State Journalarticle reporting the finding, Emily Gilbert, a postdoctoral fellow at NASA’s Jet Propulsion Laboratory in Southern California leading the project, stated:
This is one of only a few systems with multiple, small, habitable-zone planets that we know of,…That makes the TOI-700 system an exciting prospect for additional follow-up.
A closer look at the planet’s atmosphere will tell scientists more about the likelihood of life on the surface. Maybe this will be another candidate for the James Webb Space Telescope (JWST). The JWST has already probed other exoplanets to learn more about their atmosphere.
You can learn more about the new exoplanet at this NASA link.