Searching for planets that formed a long time ago in a galaxy not so far away

Title: Constraining the Planet Occurrence Rate around Halo Stars of Potentially Extragalactic Origin

Authors: Stephanie Yoshida, Samuel Grunblatt, Adrian Price-Whelan

First Author’s Institution: Department of Astronomy, Harvard University

Status: Submitted to AAS journals (open access)

Over 5,000 planets have been discovered orbiting stars in the Milky Way (MW), but so far astronomers have not confirmed detections of any planets from other galaxies. Most of our known exoplanets reside within a few kiloparsecs of the solar system (less than the distance from us to the center of the MW), so how can we find exoplanets at extragalactic distances? Today’s paper searches for planets that formed in an external dwarf galaxy and have since merged to reside in the Milky Way.

(Not) Finding Extroplanets

A few extragalactic exoplanet, or “extroplanet,” candidates have been identified in the past, though none have been confirmed to date, due to the observational difficulties of following up detections that far away. In fact, in 2010, it was announced that HIP 134044, a star left over from a small galaxy that the MW absorbed, was discovered via the radial velocity method to host a planet. Further study has since refuted this claim, noting errors in the analysis, and there is no longer evidence that such a planet exists. Today’s paper continues the streak of not discovering extroplanets, but invokes a statistical analysis to calculate how common planets may be around halo stars of extragalactic origin.

In the MW’s outer halo resides a unique population of stars with motions and elemental abundances different from stars that formed here. These stars are believed to have formed in a dwarf galaxy, referred to as Gaia-Enceladus, which merged with the MW 8-11 billion years ago. The authors used the Gaia satellite’s second data release’s measurements of stellar motions to identify stars moving in ways inconsistent with MW-formed stars. These Gaia-Enceladus stars tend to have low or negative rotational velocity in the frame of the milky way, unlike typical Milky Way stars that rotate in the disk. They also set limits on stellar magnitudes, color, and radius, combining Gaia and TESS (the Transiting Exoplanet Survey Satellite) data to select low-luminosity red giant branch stars for this study, in order to make direct comparisons to a previous study of similar stars with Kepler data. 

TESS is searching for exoplanets that pass in front of their stars, periodically causing them to appear dimmer. The authors produced light curves for their sample of 1,080 stars from TESS images and searched them visually for any of these transit dips. No planet candidates were identified, so the paper focused on using this non-detection to put a limit on how common planets could be around stars in their sample.

Upper Limits

While a non-detection may sound disappointing at first, it can actually still teach us something. By calculating a study’s “completeness,” or what fraction of such objects it could detect, a non-detection can place a constraint, or upper limit, on how common the objects may be. 

light curves for TIC TIC455692967. the left light curve just shows noise around a flux difference of 0.00. the right panel has an injected transit signal, or dip in flux at a point in time indicated with a red arrow.

Figure 1: Left: TIC455692967’s observed light curve. Right: TIC455692967’s light curve with a simulated transit signal (marked with the red arrow). (Figure 2 in the paper)

The authors used an injection-recovery method to calculate completeness. They inserted simulated transit signals into their light curves (example in Figure 1), then used a Box Least Squares search to try to identify the signals, within some precision in orbital period and transit depth. They found that roughly 30% of their injected signals were recovered, that the recovery rate was highest for planets with short periods and large radii, and that planets smaller than half Jupiter’s size were essentially undetectable.


The final upper limit calculation found that fewer than 0.52% of low-luminosity red giant halo stars should host hot jupiters (planets similar in size to Jupiter, with orbital periods of 10 days or less). This agrees with a previous estimate which put the occurrence rate at roughly 0.5%. The occurrence rate of hot jupiters correlates with stellar metallicity, generally measured as the ratio between iron and hydrogen abundances in the star. Halo stars typically have very low metallicities, suggesting that they should have ~10% the occurrence rate of hot jupiters as other stars in the Galaxy.

Period vs. planet radius table of planet occurrence upper limits. For 1-2 jupiter radius, the 3.5-10 day planet limit is 0.52%, the 10-29 day limit is 2.96%, and the 29-50 day limit is 18.2%. For 0.5 to 1 jupiter radius planets, the 3.5-10 day limit is 2.14%, the 10-29 day limit is 27.7%, and the 29-50 day limit is undefined. For 0.1-0.5 jupiter radius planets, the limits are all undefined.

Figure 2: Upper limits on planet occurrence for low-luminosity halo red giants for ranges of planet radii and orbital period. Undefined limits are the result of zero injected signals in the given radius and period range being retrieved. (Table 1 in the paper)

These upper limits are only the maximum possible occurrence rate, and these planets may be even less common than the percentages given in Figure 2. So how will we actually find these rare planets of extragalactic origin? Studying more stars! The recent 3rd data release from Gaia and ongoing TESS observations are moving astronomers in the right direction.

Edited by Sahil Hedge
Cover Image Credit: ATG Medialab/ESA/Forbes

About Macy Huston

I am a fifth year graduate student at Penn State University studying Astronomy & Astrophysics. My current projects focus on technosignatures, also referred to as the Search for Extraterrestrial Intelligence (SETI), and on microlensing searches for exoplanets.

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