Research

Signposts of the Nearest Habitable Worlds

Are we alone? How did we get here?

Exoplanet science is an explosive new field catalyzed by the discovery of over 6,000 planets orbiting other stars and recognized internationally with the award of the 2019 Nobel Prize in Physics. My colleagues and I are pushing observational techniques toward the discovery of Earth-like planets. In my research, I discover and characterize exoplanets, including measuring their masses, radii, interior compositions, orbital architectures, and host star properties, using telescopes on the ground and in space. I am investigating how thousands of known planetary systems formed, and what they reveal about the origin of our solar system and life on Earth.

One of the major questions in exoplanet science is whether life has arisen beyond our solar system. This question underpins to exoplanet community's goal to build a space telescope that would be capable of identifying spectroscopic biosignatures directly from the light of Earth-like exoplanets. However, the challenges toward this goal are significant. A key question is: where are the nearest Earth-like exoplanets? My plan is to search for potentially Earth-like planets around the nearest stars using the leading viable technique: the Doppler method. I am currently leading a two year NASA program in support of this effort using the W. M. Keck Observatory---but this is just one step in a tremendous journey that will involve many people and telescopes around the world.

As we embark on our journey to find the nearest Earths, key scientific investigations will guide our way. In particular, we need to improve our understanding of physical conditions make planets "habitable." My work has addressed this through discovering the transition between planets with rocky surfaces (good for chemistry & life) and those that are enveloped in thick layers of gas (less good for life). Our search is enriched by the fact that planets do not form in isolation, and my team has discovered over a dozen planetary siblings to rocky worlds. The dynamical interplay between Earth-like planets and their siblings is revising our leading theories of planet formation, reshaping the community's ideas of the origins and compositions of the small planets that are so prevalent in the galaxy, including which might be suitable for life.

Rocky Extra-solar Planets

The Distinction between Rocky and Gas-enveloped planets (Weiss & Marcy 2014)

Rocky surfaces are important for supporting life as we know it. Many planets are small, but which small planets are rocky? Our analysis of the masses and radii of 65 small exoplanets revealed that the rocky planets are smaller than 1.5 times the radius of Earth. This was an unexpected discovery that arose during our empirical determination of a one-to-one relationship between the masses and radii of small exoplanets (see figure, adapted from Weiss & Marcy 2014). We found that for planets larger than 1.5 Earth radii, the planet density decreases as planet radius increases, which is consistent with the planets having a gaseous envelope. However, for planets smaller than 1.5 Earth radii, planet density increases as planet size increases, in a manner consistent with the slight compression of rock. Our empirical relationship has been widely used to predict the masses of small planets both above and below the rock-to-gas transition and has guided NASA in its definition of potentially habitable planets.

Full article [link]

Jupiter-like Siblings to Earth-like Planets

The Kepler Giant Planet Search I: A Decade of Kepler Planet-host Radial Velocities from W. M. Keck Observatory

Jupiter and Saturn were important players in Earth's formation and habitability. They are thought to be responsible for the small sizes of the terrestrial planets, and Jupiter in particular slung water-bearing comets into the inner solar system, providing the material that eventually built Earth's oceans. Despite the importance of our solar system giant planets, there had not been a comprehensive observational study of how giant exoplanets correlate with the architectural properties of close-in, Earth-sized exoplanets---until our study, "The Kepler Giant Planet Search." Using the W. M. Keck Observatory High Resolution Echelle Spectrometer, we spent over a decade collecting 2844 radial velocities of 63 Sunlike stars that host 157 transiting planets. We had no prior knowledge of which systems would contain giant planets beyond 1 au, making this survey unbiased with respect to previously detected Jovians. We discovered RV-detected companions to 20 stars from our sample. These include 13 Jupiter analogs (at least 0.3 Jupiter masses at 1 au < a < 10 au), eight smaller non-transiting planets, and three stellar-mass companions. We also updated masses and densities of 84 transiting planets. This project leverages one of the longest-running and most data-rich collections of RVs of the NASA Kepler systems yet, and it will provide a basis for addressing whether giant planets help or hinder the growth of sub-Neptune-sized and terrestrial planets.

Press release [link]

Full article [link]

Systems that Have Jupiters have Irregular Spacing among their Small Planets

A postdoctoral scholar in my group, Dr. Matthias He, led a study in which he searched for patterns relating the properties of small planets close to their stars to the presence (or absence) of Jupiter-like outer planets. He found that a property of the small planets called their "gap complexity" correlates very strongly with the presence of an outer giant planet. Gap complexity is a measurement of how irregular the spacing of the planets is. Dr. He found that planetary systems that had a higher amount of irregularity in their spacing were very likely to have a giant planet (about 50% probability), whereas planetary systems with regular spacing had no associated giant planets. Our interpretation of this pattern is that the giant planets are disrupting the spacing of the planets, either knocking the planets out of coplanar alignment, moving the planets radially, or possibly ejecting the planets! More research is needed to understand the role Jupiter-like planets have played in these systems. One upshot of this project is that we now have a way to predict which systems will (or will not) have outer giant planets based on their gap complexity.

Full article [link]

Patterns in Planetary Systems

Planets Form in Patterns (Weiss et al. 2018a)

Planets in the same system tend to have similar sizes and regular orbital spacing. That is, a planet's size is very well predicted by the size of its nearest neighbor, and the orbital period ratio of a neighboring pair of planets is well predicted by the orbital period ratio of another pair of adjacent planets in the same system. The regular sizes and spacing of planets is a remnant of planet formation. New theories that reproduce the observed patterns in multi-planet systems will illuminate how a substantial fraction of planetary systems assembled.

Full article [link]

The Kepler Planets were likely Born in Multi-planet Systems (Weiss et al. 2018b)

Many of the Kepler planetary systems only have one transiting planet. Were these planets born solo, or do they have siblings?

We compared the host star properties of the systems with just one versus two or more transiting planets and found that the stellar masses, metallicities, and rotation velocities are indistinguishable between these two types of planetary architectures. Also, the planet radii from the two samples were indistinguishable*. The Kepler singles overwhelmingly have planet radii and host star properties that are indistinguishable from the properties of planets in multi-planet systems, and so it seems likely that these planets are indeed all formed in multi-planet systems. Whether the Kepler singles still have their siblings is a matter for future investigation!

Full article [link]

(*with the exception of hot Jupiters, which are known to be single, c.f. Steffen et al. (2012), Bryan et al. (2016)).

Individual planetary systems as planet formation laboratories

The present-day orbital and physical properties in individual systems open a window into the past, allowing us to test theories of planet formation. I have done several projects combining multiple datasets, including radial velocities (RVs) and transit timing variations (TTVs), to form comprehensive portraits of planet compositions and orbital configurations.

The multi-planet systems I have studied in detail include WASP-47 (top figure from Weiss et al. 2017), which has two Jupiter-mass planets and two sub-Neptunes and is the only known system in which a hot Jupiter has nearby small planets. I have also studied the Kepler-11 system, in which RVs confirmed the gas-rich compositions of the six transiting planets discovered through TTVs. In Kepler-10, a combined RV + TTV analysis revealed the likely presence of a third planet, in addition to the known rocky planet and sub-Neptune. Our use of a large RV dataset resulted in a revised composition estimate for the sub-Neptune sized planet (from "solid" to gas-enveloped).

TOI-561 is a multi-planet system containing a rocky planet with an orbital period of less than half a day. At an age of 10 billion years, the host star is one of the oldest stars in the Milky Way galaxy, making this one of the oldest rocky planets yet discovered (artistic rendition by Adam Makarenko / W. M. Keck Observatory).

Full articles [TOI-561][Kepler-88][WASP-47] [Kepler-11] [Kepler-10] [KOI-94]

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