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- Astronomers have confirmed an unusual planet called GJ 523b.<\/strong> It belongs to a rare group of worlds known as mega-Earths.<\/li>\n
- The planet is only about 2.5 times as wide as Earth but has more than 23 times its mass.<\/strong> That makes it unusually dense.<\/li>\n
- Scientists don\u2019t understand how it formed this way.<\/strong>\u00a0A planet this massive would normally be expected to have a thick atmosphere of hydrogen and helium.<\/li>\n<\/ul>\n
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Giant mega-Earth is 23 times Earth\u2019s mass<\/h3>\n
On July 23, 2026, astronomers at the University of Wisconsin\u2013Madison announced the confirmation of GJ 523b, an unusually dense mega-Earth first detected in data from NASA\u2019s TESS telescope.<\/p>\n
Rocky exoplanets called super-Earths are quite common in our galaxy. These worlds are larger and more massive than Earth, but smaller than Neptune. But now astronomers have found an even more massive planets called\u00a0 mega-Earths. Fewer mega-Earths have been discovered so far than super-Earths, but they do exist. This newest one \u2013 GJ 523b \u2013 might be the best example known so far. It is a whopping 23 times as massive as Earth.<\/p>\n
GJ 523b is 2.5 times the diameter of Earth. Some super-Earths can be that large, but the extreme mass is unusual. And the planet is estimated to be only 170 million years old. That\u2019s in contrast to Earth\u2019s age of 4.54 billion<\/em> years. So the GJ 523 system is much younger than our own solar system. The planet orbits a orange dwarf star.<\/p>\n
Thomas Beatty at University of Wisconsin-Madison said:<\/p>\n
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People have been using the phrase \u2018Mega-Earth\u2019 for more than a decade, but we\u2019ve never had a planet that let us say concretely what one is. GJ 523b finally does. What is also striking is that nailing down the definition of a Mega-Earth isn\u2019t something us astronomers can really do by ourselves: We need geologists who understand how iron and rock behave at pressures no laboratory on Earth can reach, and atmospheric scientists who can tell us how much of what we measured is rock at all.<\/p>\n<\/blockquote>\n
The new paper is still undergoing peer-review but was first published on the preprint server arXiv on March 25, 2026.<\/p>\n
![\"Diagram](\"https:\/\/earthsky.org\/upl\/2026\/08\/radius-density-mega-Earths-comparison-Kroft-et-al-March-25-2026.jpg\")
<\/p>\nTESS makes a surprising discovery<\/h3>\n
Astronomers first used NASA\u2019s TESS space telescope first detected GJ 523b as it passed in front of its star, causing a tiny dip in the star\u2019s brightness. These transits allowed astronomers to estimate the planet\u2019s size.<\/p>\n
Lead author Max Kroft at University of Wisconsin-Madison said:<\/p>\n
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There\u2019s this periodic dipping of the star\u2019s light. We think that\u2019s a planet passing in front of the star and transiting. It\u2019s blocking some of the light from the star, and the star gets dimmer.<\/p>\n
We picked out this planet based on what we thought its size and temperature were. A bigger planet makes a bigger dip, so we get an idea of the size, and based on how often that dip happens, we get the distance of its orbit and we can use that to estimate the temperature of the exoplanet.<\/p>\n<\/blockquote>\n
The researchers then used the NEID spectrograph and data on the WIYN 3.5-meter telescope to determine it\u2019s mass. High-resolution imaging from Gemini North and Palomarto were used to further study GJ 523b. They found some surprises. It was more massive than most rocky planets of its size; 23.5 times more massive than Earth and 2.5 times the diameter of Earth, or 60% the size of Neptune. As Kroft noted, this was an unexpected finding:<\/p>\n
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This isn\u2019t what we expected at all. Dense planets like this aren\u2019t uncommon, but they\u2019re usually small rocky planets similar to Earth or Mercury. This planet is 2.5 times bigger than the Earth.<\/p>\n<\/blockquote>\n
![\"Man](\"https:\/\/earthsky.org\/upl\/2026\/08\/Max-Kroft-University-of-Wisconsin-Madison.jpg\")
Max Kroft at University of Wisconsin-Madison led the new study about GJ 523b. Image via University of Wisconsin-Madison.<\/figcaption><\/figure>\n How did GJ 523b form this way?<\/h3>\n
So how did GJ 523b end up being so extremely dense and massive? Why didn\u2019t the planet develop a massive hydrogen atmosphere?<\/p>\n
In the words of Kroft, the discovery was:<\/p>\n
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a real curveball.<\/p>\n<\/blockquote>\n
During planet formation, worlds with cores around 10 to 20 times Earth\u2019s mass are generally expected to pull in thick atmospheres of hydrogen and helium. GJ 523b is massive enough for this to happen, yet it appears to contain very little gas.<\/p>\n
The study found that ordinary heating from the star probably could not remove enough gas. Giant impacts, an extremely close passage by the star or forming without much gas are possible explanations.<\/p>\n
Kroft also notes that the planet would still have been hot from its formation. Too hot to hold onto its atmosphere. Krofft explained:<\/p>\n
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It kind of blows away. A planet can\u2019t hold on to its atmosphere if it\u2019s really hot, and so you could be left with this big glob of rock made by these two planets with very little atmosphere.<\/p>\n<\/blockquote>\n
The planet\u2019s orbit offers another clue.<\/h3>\n
The researchers estimate that it is tilted by at least 71 degrees relative to the rotation of its star. This unusual alignment suggests that GJ 523b might have migrated from elsewhere in the system or experienced a turbulent event early in its history.<\/p>\n
More planets like GJ 523b<\/h3>\n
The researchers are hopeful that astronomers will find more planets like GJ 523b. There are over 6,000 confirmed exoplanets already found, and thousands more candidates. Kroft said:<\/p>\n
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It\u2019s hard to infer things about planet formation in general from a sample size of one. We\u2019re not going to get to 10,000 of these over-dense planets, but if we can get to 20 or 30, maybe some trends might pop out, where maybe the heaviest ones have shorter orbital periods, or they tend not to have companion planets.<\/p>\n<\/blockquote>\n
Bottom line: Astronomers discover mega-Earth, GJ 523b, has 23.5 times Earth\u2019s mass but only 2.5 times its radius. Its extreme density, apparent lack of a thick gas atmosphere and highly tilted orbit challenge standard ideas about how massive planets form.<\/p>\n
Source: GJ 523b is a Massive, 170 Myr-old Mega-Earth, Likely on a Polar Orbit<\/p>\n
Via University of Wisconsin-Madison<\/p>\n
Via Phys.org<\/p>\n
Read more: Kepler-10 star system has a mega-Earth!<\/p>\n
Read more: Exoplanet art lets you visualize alien worlds<\/p>\n
<\/div>\n
\n - The planet is only about 2.5 times as wide as Earth but has more than 23 times its mass.<\/strong> That makes it unusually dense.<\/li>\n