Multi-planetary system discovered with five rocky planets
It’s a bird! It’s a plane! No, it’s super-Mercury!
What are solar systems other than our own like? Astronomers at the Instituto de Astrofísica e Ciências do Espaço (Institute of Astrophysics and Space Sciences) in Portugal furthered our knowledge of that question. Recently, they discovered three super-Earths and two super-Mercuries orbiting a star, charmingly named HD 23472.
The five planets orbiting HD 23472 are only the most recent discoveries in a long line of exoplanet discoveries since the 1990s. In fact, more than 5,000 exoplanets are currently known, and our sampling of the galaxy so far has led astronomers to conclude that there are more planets than stars in the Milky Way. Discoveries have included everything ranging from hellscape worlds covered in lava and raining glass to planets so puffy they have the density of cotton candy. Aside from the few shocking discoveries, the most common planets discovered around other stars are Neptune-like planets and gas giants. So, what makes this most recent discovery of exoplanets stand out from the other 5,000?
To help me understand what the significance of these new planets is, I talked to the University of Regina’s own Dr. Samantha Lawler, an astronomer at who works jointly with Campion College and the U of R. Her recent work studying the Kuiper Belt has branched into studying debris disks around other stars. She thinks finding a system with so many planets can help us answer whether or not it is completely unique or a more average solar system.
The planets in HD 23472 were discovered using the European Southern Observatory’s Very Large Telescope, which is based in Chile. For clarity, the Very Large Telescope is the name of the telescope, not just a description of its size, but it is also an apt description of its four 8.2 metre main mirrors. One part of the telescope is a piece of equipment nicknamed ESPRESSO, which is an acronym for a special type of spectrograph that can measure very sensitive changes in the light coming from a star. ESPRESSO falls under the category of a radial velocity technique for discovering exoplanets, and according to NASA, radial velocity techniques have been used to discover over 1,000 planets.
The radial velocity technique relies on the fact that a planet is held in orbit by the pull of a star’s gravity; the planet pulls back. This means that as planets go around a star, the star wobbles ever so slightly. Since the star is moving, the light will shift ever so slightly, which sensitive instruments like ESPRESSO can pick up. The observations made in the most recent findings piggy-back off previous observations of two of the super-Earths in HD 23472 that used transit spectroscopy, a method where the light from a star that is reflected through the planet’s atmosphere is observed. The authors hope that follow-up work will use the James Webb Space Telescope to look at the system using yet another method, direct imaging. Combining multiple methods can help to make results more accurate and can also give us information about different aspects like size, mass, or composition, which can all be combined to give us a more complete picture of these strange new worlds.
The addition of so many different data points by different teams highlights the collaborative nature of science. “One thing that’s really nice about astronomy as a science is a lot of our data is public,” said Dr. Lawler. As such, the published article consists of 41 authors from 20 different institutions who work in three different continents, showing how international collaboration is essential for such discoveries.
While a super-Earth sounds remarkable, some projections estimate they could account for one-third of all exoplanets. The ‘super-’ prefix on these planets refers to the fact that they are at least twice as large as our own Earth or Mercury, but aren’t yet large enough to be put into a larger class, such as an ice-giant. What many find surprising about this discovery is actually the super-Mercuries. Including the two from this discovery, only eight have been discovered so far.
Finding other Mercury-like objects out there can help us unravel mysteries in our own solar system. For example, Mercury has a very thin mantle, unlike Earth, which has long baffled astronomers. Some have proposed fluke events like a large asteroid impact that could rip away the mantle as an explanation. However, finding similar planets in other solar systems argues for an explanation that would be more common than unique events like collisions.
Having so many rocky planets around a single star also teaches us something about our universe. Dr. Lawler said, according to these findings, “it appears that it’s really easy to form lots of rocky planets close to stars.” A boon for those who are looking for Earth-like planets out there, though the five planets from this discovery in particular are too close to their sun to be a likely candidate for life. On the other hand, we’ve also discovered some ways in which our solar system is special. Previous findings show that super-Earths are common throughout the galaxy, but we don’t have any rocky planets that big in our own solar system. Why we don’t have one ourselves remains a mystery to be solved. Further mysteries, such as how two Mercury-like planets were formed in the same system and the exact atmospheric composition of these planets, remain.
For those of you hoping to meet some friends in the final frontier, when asked about the possibility of alien life around HD 23472, Dr. Lawler replied “There could be some life forms, but still, Earth is the only place in the universe that we know for sure has life.” In fact, our next steps at identifying life on other planets will have to wait. The James Webb is scheduled to take a closer look at two other super-Earths and may one day take a closer look at the planets in HD 23472, though it isn’t scheduled to.
The suggested use of the James Webb Telescope comes shortly after the telescope made headlines this summer by finding water on an exoplanet. Although, while the James Webb can gather useful data, it isn’t a telescope built for detecting signs of life, so it has a limited capability to do so. Currently, many astronomers are waiting on three ground-based telescopes specifically designed to look for biosignatures, set to finish construction later this decade. Mirroring the aptly named Very Large Telescope, one of the three telescopes being built is the Extremely Large Telescope, and it will surpass its smaller sibling in exoplanet discovery in 2025. Clearly, we have much more exoplanet news to look forward to in the years ahead.