October 07, 2025
How do habitable environments arise and evolve within the context of their planetary systems? This is one fundamental question, and it can be addressed partly by identifying how planets in habitable zones obtain water. Historically, astronomers
considered that water was delivered to the Earth via dynamical shake-up by Jupiter, which took place during the formation and post-formation eras (e.g., \(\lesssim 100\) Myr). This hypothesis has recently been challenged by
a more dynamic view of planet formation; planet-forming materials move in protoplanetary disks via various physical processes such as pebble drift and planetary migration. Habitable Worlds Observatory (HWO) will open a new window to address this
important, but difficult question by discovering and characterizing Earth-like exoplanets around G-type stars. In this article, we consider two possible working hypotheses: (1) the abundance of water on planets in habitable zones has any
correlation with the presence of outer planets; and (2) the abundance of water on planets in habitable zones has no correlation with the presence of outer planets. We discuss what physical parameters need to be measured to differentiate these two
hypotheses and what observational capabilities are desired for HWO to reliably constrain these physical parameters.
Identifying how habitable environments emerge and evolve is one key information to quantify the ubiquity of habitable planets in the Universe. The science case described below that may be able to be explored by HWO will therefore enable astronomers to scientifically address one of the fundamental questions of humanity: “Are we alone?”
This science case is relevant to the following Key Science Questions and Discovery Areas of the Astro2020 Decadal Survey Report:
E-Q3. How do habitable environments arise and evolve within the context of their planetary systems?
E-Q3a. How are potentially habitable environments formed?
E-Q3b. What processes influence the habitability of environments?
E-Q3d. What are the key observable characteristics of habitable planets?
The origin of water on the Earth remains elusive. Historically, astronomers considered that water on the Earth was likely delivered from the asteroid and Kuiper belt regions [1]. This was motivated by the fact that the Earth is extremely water-poor (i.e., \(0.07-0.23\) wt%).
The key underlying assumptions of this hypothesis are 1) that the location of the water snow line was around \(2-3\) au at the era of planet formation and 2) that the position of Jupiter was at the current position (i.e., 5.2 au). Water could be delivered to the Earth from the formation era to the post-formation era as the form of embryos/planetesimals from the asteroid belt regions as well as the form of planetesimals from the Kuiper belt region.
There are a number of unknowns to tightly constrain the origin of water on the Earth. Below are some examples:
The composition of planet-forming materials (i.e., planetesimals and embryos) remains poorly constrained. While astronomical observations and sample return missions (e.g., Hayabusa, OSIRIS-REx) provide useful constraints [2]–[4], these constraints are not sufficient to develop a detailed statistical understanding of the properties of planet-forming materials.
The position of the snow line could move during the formation era, and the position (and eccentricity) of Jupiter could evolve with time as well. The discovery of exoplanets challenges the canonical theory of planet formation [5]–[7]. The recent dynamic view of planet formation suggests the movement of planet-forming materials via inward pebble drift and planetary migration during the formation era and poses serious questions on the two fundamental assumptions adopted in previous studies.
The lack of detections of Earth-like exoplanets around G-type stars hinders testing theoretical predictions. While detailed simulations could be run [8], the predictive power of such simulations cannot be quantified partly because the value of model parameters to be used in simulations is poorly constrained as describe above, and partly because no observations (except for the solar system) are available for testing the outcome of simulations.
The origin of water on planets in habitable zone is one important, but difficult question to address. However, HWO will open a new window. The main objective of this science case is therefore attempt to test two possible working hypotheses:
The abundance of water on planets in habitable zones has any correlation with the presence of outer planets; or
The abundance of water on planets in habitable zones has no correlation with the presence of outer planets.
A number of physical parameters are involved to test the above hypotheses. Below is the list of these parameters.
To conduct hypothesis testing, an enough number of exoplanets both in habitable zones and beyond need to be detected.
Under the assumption that 10% of candidate planets being observed that are actually Earth-like, about 30 candidates need to be observed for the robust detection of a single Earth-like planet [9]. Given that the current occurrence rate estimate of eta-Earths is about 0.3 [10], [11], at least 100 targets need to be observed.
The current occurrence rate estimate of distant massive planets by direct imaging is about a few % around 5 au [12]; if 100 targets will be observed, then about few distant planets will be discovered.
It is very challenging to reliably estimate the abundance of water in planets.
One possible approach may be to infer the water abundance from planet mass and radius. This approach may work if the water abundance is high enough that the bulk composition of corresponding planets deviates from the Earth-like composition in the mass-radius diagram.
If the water content is very small like the Earth, the above approach won’t work, and another approach is needed. One tentative approach may be to detect water vapor in exoplanet atmospheres.
If the water abundance will be inferred from planet mass and radius, then accurate determination of these two quantities is necessary. For instance, if planet mass is measured at infinite accuracy (which is not possible) and if the water mass fraction needs to be inferred within the 10 % accuracy, then the radius measurement tolerates only less than the 5% relative error, using the mass-radius relation suggested by [13].
The conventional disk model infers the position of the water snow line to be around \(2-3\) au around G-type star [14].
For this case, water delivery by dynamical shake-up would be most efficient when distant planets are located around 5 au due to gravitational resonant interaction [1], [8].
On the contrary, exoplanet observations infer that the orbital architecture of planetary systems could have huge diversity. It might thus be useful to consider two semimajor-axis bins (5-10 au and beyond) to quantify any correlations.
Two kinds of observations will be needed to achieve our science case.
The first kind of observations are imaging of planets at visible with coronagraph that will achieve:
the contrast ratio of \(\sim 4 \times 10^{-11}\),
the inner working angle (IWA) of \(< 80\) mas at 1 \(\mu\)m, i.e., \(> 2 \lambda/D\) or \(\sim 0.8\) au at 10 pc,
the outer working angle (OWA) of \(> 1\) arcsec at 0.5 \(\mu\)m, i.e., \(< 80 \lambda/D\) or \(\sim 10\) au at 10 pc, and
the astrometric accuracy of \(\sim 1\) cm s\(^{-1}\), i.e., \(\sim0.3 ~ \mu\)arcsec,
where \(D\) is the mirror diameter. The last requirement will allow measuring the planet mass of \(\sim 1 M_{\oplus}\) [15].
Future ELT observations would be complementary to discover and characterize distant planets. \(N-\)body simulations suggest that observations of planets at about eight different epochs would be needed to reliably constrain orbital parameters (e.g., semimajor-axis, eccentricity, inclination) of planets [15], [16].
The second kind are spectroscopic observations of planets in habitable zones that will achieve:
the wavelength coverage of \(0.7 - 1.5 ~\mu\)m, and
the spectral resolution of \(R > 40\).
These observations will detect at least two water lines because atmospheric water vapor has five broad spectral absorption features within the wavelength range [15].
Acknowledgements. This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). Y.H. is supported by JPL/Caltech.