A Carnegie-led survey of exoplanet candidates identified by NASA's
Transiting Exoplanets Satellite Survey (TESS) is laying the groundwork to
help astronomers understand how the Milky Way's most common planets formed
and evolved, and determine why our solar system's pattern of planetary
orbits and sizes is so unusual.
Carnegie's Johanna Teske, Tsinghua University's Sharon Wang (formerly of
Carnegie), and Angie Wolfgang (formerly of Penn State University and now at
SiteZeus), headed up the Magellan-TESS Survey (MTS), which is halfway
through its three-year planned duration. Their mid-survey findings, in
collaboration with a large, international group of researchers, will be
published in the Astrophysical Journal Supplement Series.
NASA's Kepler Mission revealed that our galaxy is teeming with
planets—discovering thousands of confirmed worlds and predicting that
billions more exist. One of the surprises contained in this bounty is that
exoplanets between the size of Earth and Neptune are the most common
discovered so far, despite the fact that none exist in our own solar system.
These "in between" planets appear to come in two distinct sizes—roughly one
to 1.7 (super-Earths) and roughly two to three (mini-Neptunes) times the
size of the Earth—indicating different gas content in their compositions.
"We want to understand whether super-Earths and mini-Neptunes were distinct
from their earliest origins, or whether some aspect of their evolution made
them deviate from each other," Teske explained. "In a sense, we are hoping
to probe the nature-nurture question for the galaxy's most common
exoplanets—were these planets born differently, or did they diverge due to
their environment? Or is it something in between?"
The survey is using TESS data and observations from the Magellan telescopes
at Carnegie's Las Campanas Observatory in Chile to study a selection of 30
small, relatively short-period planet candidates. The TESS data show dips in
brightness when an object passes in front of its host star. The amount of
dimming allows the survey team to measure the radius of a planet candidate.
This information is combined with observations gathered by the Planet Finder
Spectrograph at Las Campanas that works by using a technique called the
radial velocity method, which is currently the most common way for
astronomers to measure the masses of individual planets.
The Magellan-TESS survey team is interested in the interplay between key
variables that could help astronomers better characterize the formation
pathways of super-Earth and mini-Neptune planets. They are looking for
trends in the relationships between a planet's mass and its radius; the
properties of its host star, including composition and the amount of energy
it radiates onto the planet; and the architecture of the planetary system of
which it the planet a member.
"The underlying relationship between radius and mass for these small planets
is crucial to figuring out their general compositions, via their overall
density, as well as how much variation there is in their compositions,"
explained Wolfgang. "Quantifying this relation will help us discern whether
there is one formation pathway or multiple avenues."
What sets this survey apart from prior work is its scope—the team designed
the survey from the start to try to account for biases that could skew how
the results are interpreted in a broader context. Their goal is to be able
to draw robust conclusions about super-Earths and mini-Neptune planets as a
population, versus just a collection of 30 individual objects.
The mid-survey findings, which represent a significant contribution to the
number of small planets with known masses and radii, already hint at
evidence for small observational selection biases that may have affected
scientists' work on mass measurements. The MTS could thus provide an
important framework for future radial velocity studies of transiting
planets.
Looking forward, the next half of the survey will focus on completing the
sample—this paper contains 22 of the planned 30 candidates—as well as
continuing to monitor all the systems for longer-period planets not detected
by TESS to probe system architectures. Checking the influence of the host
star composition is another next step, since past work has suggested that
the compositions of planets may be related to those of the stars they orbit.
"We hope that gaining this multidimensional understanding will significantly
improve our knowledge of exoplanet evolution, and perhaps explain why our
own solar system seems unusual," Wang concluded.
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