(ORDO NEWS) — Around an orange dwarf just 130 light-years from Earth, astronomers have discovered an unexpected treasure.
Not only are three rocky super-Earth worlds orbiting the star, but two more exoplanets in systems are almost incredibly rare in our records.
These two are super-Mercuries, a type of exoplanet that is so hard to detect that we’ve only identified eight, including new discoveries.
All five exoplanets are too close to their host star for life, as we know it’s possible, but the discovery represents the best laboratory for studying super-Mercury exoplanets – and Mercury itself, right here in the solar system.
“For the first time, we have detected a system with two supermercuries,” says astrophysicist Susana Barros of the Institute of Astrophysics and Space Sciences (IA) in Portugal. “This allows us to get clues about how these planets formed, which could help us rule out some possibilities.”
Finding exoplanets is hard, and finding small ones is even harder. Astronomers currently rely on two main methods: the transit method and the radial velocity method.
For the transit method, astronomers will look for very faint, regular dips in the star’s light a sign of an orbiting exoplanet passing between us and it.
The radial velocity method looks for changes in the wavelengths of light reaching us from a star as it “wobbles” in place, dragged along by the gravitational pull of an orbiting exoplanet. .
As you understand, both of these signals – the passage and the radial velocity – are tiny. We are more likely to detect stronger signals from larger exoplanets.
NASA‘s TESS Exoplanet Search Telescope, using the transit method, first detected two exoplanets orbiting the star HD 23472 a few years ago, and then observations confirmed their presence. Two other exoplanet candidates have also been discovered.
Barros and her team wanted to take a closer look at the HD 23472 system because they were trying to understand the difference in minor planet radii: the mysterious absence of planets between 1.5 and 2 Earth radii. The two confirmed exoplanets were on the larger side of this gap, and the two candidates were on the smaller side.
Astronomers suspect the difference may be the presence or absence of an atmosphere. This can be done by calculating the exoplanet’s density if you have transit and radial velocity data.
Transit data, which tells you how much of a star’s light is being blocked by an exoplanet, can give you its size. The radial velocity data, which tells you about the gravitational pull of a star from an exoplanet, can give you information about its mass. Density can be calculated using these two measurements.
So, between July 2019 and April 2021, the team set about getting very precise measurements of the star’s radial velocity using the ESPRESSO spectrograph at the European Southern Observatory’s Very Large Telescope. . And they found evidence that there is a fifth exoplanet near the star with less mass than Earth.
Then, in October 2021, TESS recorded the transit signature of this fifth exoplanet.
The team processed all the numbers and characterized the system. From closest to star to farthest:
- HD 23472 d has an orbital period of 3.98 days, a radius 0.75 times that of Earth, and a mass 0.54 times that of Earth.
- HD 23472 e, the most recent discovery, has a period of 7.9 days, 0.82 Earth radii, and 0.76 Earth masses.
- HD 23472 f has a period of 12.16 days and hours of 1.13 Earth radii and 0.64 Earth masses.
- HD 23472 b has an orbital period of 17.67 days and is 2.01 Earth radii and 8.42 Earth masses.
- HD 23472 c has an orbital period of 29.8 days and is 1.85 Earth masses and 3.37 Earth radii.
These measurements give Earth-like densities for the three outer exoplanets and are consistent with significant atmospheres.
The two inner exoplanets, however, have a high density. This suggests that they may be similar to Mercury in composition, with a large core and small mantle compared to other planets.
We don’t know why Mercury is like this; maybe it hit something early in the solar system that literally threw a bunch of material off, or that the heat from the Sun vaporized some of it.
Finding two together suggests that one an event such as a collision might be unlikely.
“If a collision strong enough to create a super-Mercury is already very unlikely, two giant collisions in the same system seem very unlikely,” explains Barros.
“We still don’t know how these planets formed, but it looks like it has to do with the composition of the parent star. This new system can help us figure that out.”
It is not yet clear whether the two candidates for super mercury have atmospheres; we’ll need a more powerful telescope to find out.
“Understanding how these two supermercuries formed will require further characterization of the composition of these planets,” says IA astronomer Olivier Demangeon.
“Because the radius of these planets is smaller than that of the Earth, modern instruments are not sensitive enough to determine the composition of their surface or the presence and composition of a potential atmosphere.”
With the number of large telescopes currently under construction, hopefully we won’t have to wait long.
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