# What is the densest known exoplanet?

55 Cancri e is said to be 60 percent larger in diameter than Earth but have 8 times, the mass, making it twice as dense as Earth, and almost as dense as lead.

https://www.space.com/11544-densest-alien-planet-55cancrie.html

According to those figures it would have 8 times the mass of Earth in 4.096 times the volume, or 1.953125 times Earth's density. Since the density of Earth is 5.514 grams per cubic centimeter, 55 Cancri e should have a density of 10.769531 grams per cubic centimeter. Lead has a density at room temperature of about 11.34 grams per cubic centimeter.

According to Wikipedia:

The mass of the exoplanet is about 8.63 Earth masses and its diameter is about twice that of the Earth,2 thus classifying it as the first super-Earth discovered around a main sequence star, predating Gliese 876 d by a year.

https://en.wikipedia.org/wiki/55_Cancri_e

With 8.396 Earth mass and twice the diameter of Earth 55 Cancri e would have 1.0495 times the density of Earth, or 5.7869 grams per cubic centimeter.

The radial velocity method used to detect 55 Cancri e obtains the minimum mass of 7.8 times that of Earth,2 or 48% of the mass of Neptune. The transit shows that its inclination is about 83.4 ± 1.7, so the real mass is close to the minimum. 55 Cancri e is also coplanar with b.

And the density is given as 6.66 grams per cubic centimeter, giving this as the source:

Planet GJ 367 b is also considered to be very dense.

They found that the exoplanet has a radius about 72 per cent of Earth’s and a mass just over half our planet’s mass. With those measurements, they were able to predict its interior structure – it probably has an iron core that takes up about 86 per cent of its radius, similar to Mercury’s strangely huge iron core.

https://www.newscientist.com/article/2299504-extremely-dense-planet-with-an-enormous-iron-core-has-an-8-hour-year/

According to those rough figures, the planet would have at least 0.5 of Earth's mass in about 0.373248 of Earth's volume, and thus about 1.33959 of Earth's density - 7.3865 grams per cubic centimeter. Iron has a density of 7.874 grams per cubic centimeter.

According to NASA, GJ 367 b has a mass of 0.546 Earths and a radius of 0.718 Earth, which would give it 0.3701462 the volume of Earth and a density of 1.475 Earth or about 8.13366 grams per cubic centimeter.

https://exoplanets.nasa.gov/exoplanet-catalog/8049/gj-367-b/

This report says:

GJ 367b has a radius of 0.718 ± 0.054 Earth-radii and a mass of 0.546 ± 0.078 Earth-masses, making it a sub-Earth planet. The corresponding bulk density is 8.106 ± 2.165 grams per cubic centimeter—close to that of iron.

https://www.science.org/doi/10.1126/science.aay3253

Wikipedia's list of exoplanet extremes lists the densest exoplanet as Kepler-131 c, with a highly uncertain density of 77.7 plus or minus 55 grams per cubic centimeter - between 22 and 132 grams per cubic centimeter.

https://en.wikipedia.org/wiki/List_of_exoplanet_extremes#Planetary_characteristics

It gives the source as:

https://arxiv.org/abs/1401.4195

On page 23 they discuss the two planets of Kepler-131:

Kepler identified two transiting planets with orbital periods of 16.0 and 25.5 d and radii 2.4 and 0.8 R⊕

The RV measurements for Kepler-131 span 741 days (Figure 30, top) and are listed, along with their log R′ HK activity values in Table 18. The results consist of an RV detection of the 16 d planet giving a mass of 16.1 ± 3.5 M⊕, and a density of 6.0 ± 2.0 g cm−3 . We find a similar mass and density whether we fit this planet by itself or with the second planet simultaneously. The second planet shows up in the RVs only marginally. A self-consistent fit of RVs and photometry yields a peak of the posterior mass distribution at M = 8.25 ± 5.9 M⊕ and density of 78 ± 55 g cm−3 . The density is unphysically large, and hence the mass is too large, indeed a detection at less than 2σ. We find an upper limit to the mass of Kepler-131c from the 95th percentile of the posterior mass distribution, yielding M < 20.0 M⊕ . Certainly the mass of Kepler-131c remains highly uncertain. Table 2 lists all of the best-fit planet parameters.

Kepler-131 c should have a radius of about 0.8 Earth & thus a volume of about 0.512 Earth and so with a mass of 8.25 plus or minus 5.9 times the mass of Earth it should have a density between about 4.5898 and 27.6368 times Earth's density, 25.308 to 152.388 grams per cubic centimeter.

And since the calculated density of Kepler-131 c was so unbelievably large, I have to wonder if anyone has tried to measure its radius and/or mass again since 2014.

So what is the densest known exoplanet now in 2022, and are there any known exoplanets which might possibly have higher densities?