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| Main Authors: | , , , , , , , , , , , , , , , , , , , , |
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| Format: | Preprint |
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2026
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| Online Access: | https://arxiv.org/abs/2603.24682 |
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| _version_ | 1866917362169544704 |
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| author | Kroft, Maxwell A. Beatty, Thomas G. Salzer, Joseph M. Zwicker, Claire Triantafillides, Anastasia Becker, Juliette Soares-Furtado, Melinda Cisewski-Kehe, Jessi Lissauer, Jack J. Armitage, Tayt S. Livesey, Joseph R. Narayan, Ritvik Sai Weaver, Susanna Widicus Zhang, Ke Bieryla, Allyson Ciardi, David R. Clark, Catherine A. Felsmann, Miranda Fernandes, Rachel B. Howell, Steve B. Lund, Michael B. |
| author_facet | Kroft, Maxwell A. Beatty, Thomas G. Salzer, Joseph M. Zwicker, Claire Triantafillides, Anastasia Becker, Juliette Soares-Furtado, Melinda Cisewski-Kehe, Jessi Lissauer, Jack J. Armitage, Tayt S. Livesey, Joseph R. Narayan, Ritvik Sai Weaver, Susanna Widicus Zhang, Ke Bieryla, Allyson Ciardi, David R. Clark, Catherine A. Felsmann, Miranda Fernandes, Rachel B. Howell, Steve B. Lund, Michael B. |
| contents | We use WIYN/NEID radial velocity measurements to confirm the planetary nature and measure the mass of the TESS transiting exoplanet candidate around the mid-K dwarf GJ 523 ($V=9.23$, $K=6.525$). We find that GJ 523b is on a 17.75 day orbit and has a radius of $2.55\pm0.15\,R_\oplus$, a mass of $23.5\pm3.3\,M_\oplus$, and a zero-albedo equilibrium temperature of 538 K. GJ 523b's high bulk density of $7.8\pm1.8$ g cm$^{-3}$ and position on a mass-radius diagram implies a surprising low atmospheric mass fraction despite its relatively large mass. Additionally, we determine that the system has an age of $169^{+100}_{-48}$ Myr through a gyrochronological analysis of GJ 523 and its comoving companions. We also use the SED-derived stellar radius, the photometric rotation period, and the spectroscopic $v\sin i_\star$ to derive a stellar inclination of $17.6\pm5.0$ degrees, implying that GJ 523b has a minimum orbital obliquity of $71.4_{-5.0}^{+4.7}$ degrees. GJ 523b's high mass, apparent lack of a gas envelope, young age, and high orbital obliquity present a challenge to typical planet formation pathways, and at the moment there is not enough data on the system to definitively determine how GJ 523b formed. Finally, we present a new observational classification for ultra-dense, sub-Neptune-sized exoplanets similar to GJ 523b: the mega-Earths, planets with $R_p \geq2.1\,R_\oplus$ and $ρ_p \geq 5.5$ g cm$^{-3}$. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2603_24682 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | GJ 523b is a Massive, 170 Myr-old Mega-Earth, Likely on a Polar Orbit Kroft, Maxwell A. Beatty, Thomas G. Salzer, Joseph M. Zwicker, Claire Triantafillides, Anastasia Becker, Juliette Soares-Furtado, Melinda Cisewski-Kehe, Jessi Lissauer, Jack J. Armitage, Tayt S. Livesey, Joseph R. Narayan, Ritvik Sai Weaver, Susanna Widicus Zhang, Ke Bieryla, Allyson Ciardi, David R. Clark, Catherine A. Felsmann, Miranda Fernandes, Rachel B. Howell, Steve B. Lund, Michael B. Earth and Planetary Astrophysics We use WIYN/NEID radial velocity measurements to confirm the planetary nature and measure the mass of the TESS transiting exoplanet candidate around the mid-K dwarf GJ 523 ($V=9.23$, $K=6.525$). We find that GJ 523b is on a 17.75 day orbit and has a radius of $2.55\pm0.15\,R_\oplus$, a mass of $23.5\pm3.3\,M_\oplus$, and a zero-albedo equilibrium temperature of 538 K. GJ 523b's high bulk density of $7.8\pm1.8$ g cm$^{-3}$ and position on a mass-radius diagram implies a surprising low atmospheric mass fraction despite its relatively large mass. Additionally, we determine that the system has an age of $169^{+100}_{-48}$ Myr through a gyrochronological analysis of GJ 523 and its comoving companions. We also use the SED-derived stellar radius, the photometric rotation period, and the spectroscopic $v\sin i_\star$ to derive a stellar inclination of $17.6\pm5.0$ degrees, implying that GJ 523b has a minimum orbital obliquity of $71.4_{-5.0}^{+4.7}$ degrees. GJ 523b's high mass, apparent lack of a gas envelope, young age, and high orbital obliquity present a challenge to typical planet formation pathways, and at the moment there is not enough data on the system to definitively determine how GJ 523b formed. Finally, we present a new observational classification for ultra-dense, sub-Neptune-sized exoplanets similar to GJ 523b: the mega-Earths, planets with $R_p \geq2.1\,R_\oplus$ and $ρ_p \geq 5.5$ g cm$^{-3}$. |
| title | GJ 523b is a Massive, 170 Myr-old Mega-Earth, Likely on a Polar Orbit |
| topic | Earth and Planetary Astrophysics |
| url | https://arxiv.org/abs/2603.24682 |