Four to five planets are possible in a planetary system located 35 light years from Earth. One of these planets is located in the star's habitable area. Another one, if it exists, is the lightest-weight known to be found using radial velocity. It only has 40% of Earth's weight. This is pretty cool.
A red dwarf named L98-59 is the host star. It is significantly smaller, cooler and dimmer that the Sun. It shines only 1% brighter than our own star. It was discovered that three planets orbit it. The transit method is used to determine if any planets pass directly in front of the star. This works only for systems that are nearly edge-on from Earth.
Zoom In Artwork that compares the sizes of Mars, Earth and the planets orbiting L 98-59. Although we don't know the actual appearance of the planets, we do know their masses and sizes. Credit: NASA Goddard Space Flight Center
These planets were first announced in 2019. In 2019, astronomers have re-observed the star with different telescopes in order to better understand the planets. You can calculate the planet's size using the transit method (the larger the planet, the darker it will appear due to more star blocking), but the mass is what we need in order to better understand them. The density (mass/volume) tells you about the planet's composition.
This is the radial velocity method. The planet orbits the star and its gravity pulls at the star. The star circles around the planet in a smaller circle than the one that the planet forms around the centre of mass. This means that half of the orbit is heading toward us while the other half is moving away. We can measure this using the Doppler shift.
It is difficult because the star's velocity can be very slow. However, it is possible to break the star's light into a spectrum and measure the results. The problem is made worse by the fact that multiple planets can have multiple shifts.
The results are well worth the effort. The team was able to see the previously identified planets (L98-59b-c and d in order from the star), but also strong evidence for a fourth planet called L9859e and hints of another.
Zoom In Comparison of the L98-59 system with ours (bottom). The x-axis refers to temperature and not distance. This highlights the possibility that the fifth planet (unconfirmed yet) may have Earth-like temperatures. The masses and radii of the fourth and fifth planets are not yet known. Credit: ESO/L. Calada/M. Kornmesser (Acknowledgment by O. Demangeon
This last one is still to be confirmed. However, if it is real, they find that it has a mass at least 2.5x Earth's and takes 23 Earth days to orbit its star (the length the star's year). It may be a rocky planet if it is only slightly larger than Earth. It orbits around 15 million kilometers from its star, which is only 1/10th of the distance Earth travels from the Sun. However, the star receives approximately the same amount heat as Earth, so its temperature could be very close to ours. This is quite interesting! It doesn't transit so it is not known its exact size.
If it is possible to exist, it would be a good candidate for being similar to Earth. It might look like Venus, if it has thick atmosphere. But let's not forget about it. But still.
Zoom In Art shows the exoplanet L9859b. It has 0.4 times Earth's mass, making it the lowest-mass planet with its mass calculated using the radial velocity method. Credit: ESO/M. Kornmesser
They also obtained much more accurate measurements of the outer three planets. L98-59b was the smallest, measuring 0.85 times the diameter of Earth and only 0.4 times the mass of ours. This gives it a density that is about 65% higher than Earth's. Its size means that it is rocky, and may not have an iron core as large. It is located approximately 3.3 million km from the star. This makes it hot. Ouch.
This is important nonetheless! It is much easier to measure the mass of a planet by using the radial velocity technique if it is larger than the star. A bigger planet tugs at its star harder. It is very difficult to measure a planet with 0.4 Earth masses, or about half the mass of Venus. This is the lowest mass confirmed planet.
This is a good thing, as a planet such as Earth orbiting further out than a star such the Sun has a lower velocity, which makes them harder to find. These measurements can be pushed quite far, as shown by the L98-59b results.
We have only been able to determine the masses and diameters of at least two planets in half-a dozen planetary systems. For those who study comparative planetaryology, it is beneficial to find multiple planets around the same star with different characteristics. These planets can be assumed to have formed around the same star at the same moment, which eliminates one uncertainty when comparing them.
I also love the miniature red dwarf systems, such as TRAPPIST-1. They are so much more different than ours, as all the rocky planets are so close to their star. They may be the most common system in the Universe. Dim bulb red dwarfs are a great place to start if you want to discover life Out There.