---
title: 'HD 137010 b: Earth-Sized Exoplanet Candidate'
url: https://www.emergentmind.com/topics/hd-137010-b
type: topic
---

# HD 137010 b: Earth-Sized Exoplanet Candidate

HD 137010 b is a cool, Earth-sized exoplanet candidate, identified via a single high signal-to-noise transit detected in 2017 with K2 Campaign 15 photometry. Orbiting the relatively bright, nearby K3.5 V dwarf HD 137010 ($V=10.14$), it exhibits a radius closely matching Earth’s ($R_p = 1.06^{+0.06}_{-0.05}\,R_\oplus$) and receives only $\sim$0.29 times Earth's insolation, placing it near the outer edge of the classical habitable zone. Its transit, depth, and photometric context make HD 137010 b the first such planet candidate transiting a Sun-like star bright enough ($V\approx10$) to enable in-depth future follow-up investigations [2601.19870].

## 1. Host Star Properties and Context

HD 137010 is a K3.5 V star with a well-characterized set of stellar parameters critical for transit and habitability analysis:

| Parameter            | Value                                | Reference/Method                    |
|----------------------|--------------------------------------|-------------------------------------|
| Spectral Type        | K3.5 V                               | Gray et al. 2006                    |
| Visual Magnitude     | $V = 10.14 \pm 0.05$ mag             | Tycho-2                             |
| Mass ($M_*$)         | $0.726 \pm 0.017\,M_\odot$           | MIST Isochrone Fit                  |
| Radius ($R_*$)       | $0.707 \pm 0.023\,R_\odot$           | MIST Isochrone Fit                  |
| Effective Temperature ($T_{\rm eff}$) | $4770 \pm 90$ K        | Spectroscopy                        |
| Stellar Density ($\rho_*$) | $2.90^{+0.29}_{-0.26}$ g cm$^{-3}$ | Transit Fit                         |
| Surface Gravity      | $\log g = 4.60 \pm 0.03$ (cgs)       | MIST Isochrone Fit                  |
| Age                  | 4.8–10 Gyr                           | Kinematics, Magnetic Activity       |

The stellar environment is photometrically quiet, with low magnetic activity ($\log R'_{{\rm HK}} \approx -4.84$), and its age is constrained by kinematics and activity indices [2601.19870].

## 2. Transit Detection and Validation

HD 137010 b was detected as a single, 10-hr-long transit in 88 days of K2 long-cadence photometry (29.4 min integration). The event is shallow ($\delta=225\pm10$ ppm) but robustly detected due to exceptionally low photometric noise (CDPP$_{6.5\,\mathrm{hr}} \approx 8.5$ ppm). The signal-to-noise ratio (SNR) for white noise was $\sim30$, with red-noise SNR between 11.2–13.

Comprehensive validation included:

- **Systematic Detrending**: Simultaneous modeling of K2 roll systematics and transit signal, following procedures from Vanderburg & Johnson (2014).
- **Neighbor and Centroid Checks**: Exclusion of variable or contaminant sources within $5'$ and centroid shifts within 1 pixel.
- **Archival and Speckle Imaging**: No background stars detected within the photometric aperture down to $\Delta K_p\sim9$ mag. Speckle imaging (Zorro, 562/832 nm) ruled out companions $>$0.1 $M_\odot$ beyond 25 AU.
- **Radial Velocity (RV) and Astrometry**: No evidence for stellar-mass companions or binaries to $\sim1\,\mathrm{m\,s}^{-1}$ from HARPS RVs and Hipparcos-Gaia astrometry.

False-positive scenarios—including eclipsing binaries, background blends, or hierarchical triples—are strongly disfavored [2601.19870].

## 3. Planetary Parameters and Orbital Solution

Fitting assumptions included circular orbits ($e=0$), quadratic limb-darkening (priors from Claret 2018 in the Kepler band), and Gaussian priors on stellar mass and density (MIST isochrones). MCMC analysis (emcee, 50 walkers, $10^8$ steps, with $P<2000$ d) yielded the following planet properties:

| Parameter          | Value                                  | 68% Confidence Interval         |
|--------------------|----------------------------------------|-------------------------------|
| Radius ($R_p$)     | $1.06\,R_\oplus$                       | $^{+0.06}_{-0.05}$ $R_\oplus$ |
| Period ($P$)       | $355$ days                             | $^{+200}_{-59}$ days          |
| Semi-major Axis ($a$) | $0.88$ AU                              | $^{+0.32}_{-0.10}$ AU         |
| $a/R_*$            | $270$                                  | $^{+93}_{-37}$                |
| Incident Flux ($I$)  | $0.29\,I_\oplus$                      | $^{+0.11}_{-0.13}\,I_\oplus$  |
| Equilibrium Temperature ($T_\mathrm{eq}$, $\alpha=0$) | $205$ K | $^{+17}_{-28}$ K |
| Equilibrium Temperature ($T_\mathrm{eq}$, $\alpha=0.3$) | $188$ K | $^{+16}_{-25}$ K |

Transit duration is $T_D = 9.76^{+0.21}_{-0.18}$ hr [2601.19870].

Key relationships underpinning the fit include:

- Transit depth: $\delta = (R_p/R_*)^2$
- Duration-stellar density relation:
$$
T_D = \frac{P}{\pi}\arcsin\left[\frac{\sqrt{(1+R_p/R_*)^2-b^2}}{a/R_*}\right]
$$
- Kepler’s third law (for $e=0$): $P^2 = \frac{4\pi^2\,a^3}{G M_*}$
- Period prior for single transit detection: $p(P) \propto P^{-5/3}$

## 4. Statistical Methodology and Model Assumptions

Analysis employed a statistical framework tailored to the single-transit regime [Kipping 2018; Sandford & Kipping 2019]. The eccentricity was considered negligible, motivated by the observed properties of small, long-period planets [Kipping et al. 2025]. Limb-darkening parameters were drawn from population priors and fit using uninformative transforms [Claret 2018; Kipping 2013]. Priors on stellar mass and density were Gaussian, derived from MIST isochrones and the latest calibrations [Dotter 2016; Choi 2016; Tayar et al. 2022].

False-positive probability constraints leveraged radial velocity non-detections, high-resolution imaging, and transit morphology (shape tests sensu Kunimoto 2025). Only periods $P < 2000$ d were permitted for MCMC convergence. The RV semi-amplitude expected for an Earth-mass planet is $K \sim 0.13\,\mathrm{m\,s}^{-1}$, at the threshold of current or next-generation ePRV capabilities.

## 5. Habitability Prospects and Climate Inference

HD 137010 b’s estimated incident flux ($0.29\,I_\oplus$) places it near the outer edge of canonical habitable-zone (HZ) boundaries [Kopparapu et al. 2013]. Specifically:

- **Conservative HZ ([1.00, 0.30] $I_\oplus$)**: 40% of posteriors fall within.
- **Optimistic HZ ([1.60, 0.27] $I_\oplus$)**: 51% of posteriors within.

With an equilibrium temperature well below the water freezing point ($T_{\mathrm{eq}} \leq 205$ K at $\alpha=0$), surface habitability requires substantial greenhouse warming (e.g., 200–500 mbar CO$_2$; Bolmont et al. 2014). A “snowball” scenario is plausible at lower atmospheric CO$_2$ or higher albedo ($T_{\mathrm{eq}} \approx 173$ K at albedo 0.5; Del Genio et al. 2019). Planet size and semimajor axis closely resemble Earth or Mars, but incident flux is significantly lower than that of Earth.

## 6. Follow-up Opportunities and Observational Outlook

HD 137010 b’s host brightness ($V=10.1$) permits the following follow-up avenues:

- **Radial Velocity:** The expected $K \sim 0.13\,\mathrm{m\,s}^{-1}$ is at the limit of near-future extreme-precision RV efforts (see EPRVWG 2021).
- **Transit Re-observation:** The probability of re-observing a transit in TESS Sector 91 was $\sim7\%$; further opportunities exist with CHEOPS and coordinated campaigns for ephemeris refinement.
- **Direct Imaging:** The planet–star separation ($\lesssim 20$ mas) is too small for coronagraphy, but future interferometric missions (e.g., LIFE; Quanz et al. 2022) could in principle resolve it.
- **Transmission Spectroscopy:** Host star brightness is favorable, but the transit depth (225 ppm) requires extremely large telescopes for atmospheric analysis.

Securing additional transits and achieving ultra-precise RV mass determinations would establish HD 137010 b as a benchmark for terrestrial planet atmospheric characterization around K-dwarfs.

## 7. Comparative Metrics and Significance

Relative to other known exoplanets, HD 137010 b is the first candidate with Earth-like dimensions and orbital period transiting a Sun-like star of sufficient brightness to enable detailed characterization [2601.19870]. Its position near the outer habitable zone and transit-derived properties make it a cornerstone for future studies of terrestrial planet formation, occurrence rates, and climatic evolution around subsolar-mass stars. Achieving repeated transit observations and next-generation RV mass measurements would transition the object from candidate to a reference archetype for exoplanetary science.

Source: https://www.emergentmind.com/topics/hd-137010-b