In the realm of Star Wars, alien heroes confront villains wielding planet-destroying superweapons “a long time ago in a galaxy far, far away.” But what do scientists truly understand about alien planets in galaxies far beyond our own? These fascinating worlds are known as extragalactic exoplanets. Assuming the Milky Way galaxy is akin to other galaxies, it is predicted to harbor similar worlds. Yet, many galaxies remain distant, rendering modern exoplanet observation techniques inadequate for their detection.

Recently, astronomers studied a stream of over 700,000 stars potentially absorbed by the Milky Way from the Sagittarius Dwarf Galaxy. Given their remoteness, the team investigated if any of these stars hosted large exoplanets orbiting close to Earth, specifically hot Jupiters, which are generally easier to identify.

The researchers established three criteria for narrowing down their star selection. First, each star must be sufficiently bright as observed by a transiting exoplanet probe like TESS to ensure high accuracy in their data processing software. Second, each star should possess at least a 50% likelihood of originating from the Sagittarius Dwarf Galaxy, based on motion and position measurements from the Gaia mission. Finally, the radius of each star needed to be less than twice that of the Sun to simplify the search for planets around smaller stars. These criteria helped refine the list to approximately 20,000 candidate stars.

Subsequently, the team utilized a software package to analyze public TESS catalog data, specifically the Eleanor TESS-Gaia light curve, also known as TGLC. Using these tools, they plotted the brightness of each star over time on a graph termed the light curve. The astronomers searched for periodic brightness dips, indicating an exoplanet passing in front of the star. This process eliminated several thousand stars affected by optical interference, refining their sample size to just over 15,000 stars.

To detect hot Jupiters, the team looked for brightness dips occurring at intervals of 14 hours to 10 days, the typical orbital periods for these planets. They employed geometric calculations to derive the radius of each exoplanet based on the proportion of starlight obscured. Candidates with dips corresponding to objects with radii more than twice that of Jupiter were excluded, as these dips likely resulted from orbiting companion stars rather than true exoplanets.

Among the examined stars, the most promising candidate for a hot Jupiter was identified as TIC 92223525. The researchers estimated that it could host an exoplanet with a radius 1.76 times that of Jupiter and an orbital period of 7.2 days. However, closer inspection of the star’s light curve indicated contamination from a neighboring star, TIC 92223526. The periodic brightness variations from this companion star mimicked those of an exoplanet, leading to a false positive in the initial screening. Consequently, the research team had to dismiss this candidate, leaving no confirmed exoplanets from their study.

The researchers drew significant conclusions from their inability to identify hot Jupiters within the Sagittarius Dwarf star sample. They estimated that if more than 1% of these stars hosted hot Jupiters, detecting at least one in a sample of over 15,000 stars would have been very probable. This limits the occurrence rate of hot Jupiters to around 1%. If accurate, future exoplanet-hunting endeavors may necessitate exploring over 11,000 stars to discover one, with reasonable scientific uncertainty suggesting at least 80,000 stars may need to be examined.

While this survey of the Sagittarius Dwarf yielded no conclusive results, the research team encourages subsequent researchers to continue their exploration of the Sagittarius Dwarf and other star streams from different galaxies. Scientists have identified over 20 such streams within the Milky Way. Investigations into these stellar streams may lead to the discovery of the first extragalactic planets or offer insights into whether other galaxies produce fewer hot Jupiters than our own. Let’s hope none of them stumble upon an extragalactic Death Star!

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