The Habitable Zones of Pre-Main-Sequence Stars (1412.1764v1)

Abstract: We calculate the pre-main-sequence HZ for stars of spectral classes F to M. The spatial distribution of liquid water and its change during the pre-main-sequence phase of protoplanetary systems is important in understanding how planets become habitable. Such worlds are interesting targets for future missions because the coolest stars could provide habitable conditions for up to 2.5 billion years post-accretion. Moreover, for a given star type, planetary systems are more easily resolved because of higher pre-main-sequence stellar luminosities, resulting in larger planet to star separation for cool stars than is the case for the traditional main-sequence (MS) habitable zone (HZ). We use 1D radiative-convective climate and stellar evolutionary models to calculate pre-main-sequence HZ distances for F1 to M8 stellar types. We also show that accreting planets that are later located in the traditional MS HZ orbiting stars cooler than a K5 (including the full range of M-stars) receive stellar fluxes that exceed the runaway greenhouse threshold, and thus may lose substantial amounts of water initially delivered to them. We predict that M-star planets need to initially accrete more water than Earth did or, alternatively, have additional water delivered later during the long pre-main-sequence phase to remain habitable. Our findings are also consistent with recent claims that Venus lost its water during accretion.

• The paper analyzes the habitable zones (HZ) of pre-main-sequence (pre-MS) F–M stars using 1-D radiative-convective climate and stellar evolutionary models.
• It finds that pre-MS HZs are significantly larger than main-sequence HZs, placing planets further out and potentially maintaining habitability longer, though cooler stars face substantial water loss risks.
• The research highlights that M-star planets likely require large initial water endowments and are promising observational targets for next-generation telescopes due to extended pre-MS phases and larger separations.

Analyzing the Habitable Zones of Pre-Main-Sequence Stars

This paper provides an extensive analysis of the habitable zones (HZ) during the pre-main-sequence (pre-MS) phase for stars of spectral classes F–M. The paper underscores the significance of this phase in determining the potential habitability of planets orbiting these stars, emphasizing the changes in spatial distribution of liquid water and its profound implications on planetary habitability.

Summary of Findings

Using 1-D radiative-convective climate and stellar evolutionary models, the authors calculated the distances of pre-MS HZs for various stellar types, spanning F1 to M8. Notably, the pre-MS HZ is observed to be significantly larger than the traditional main-sequence (MS) HZ for these stars. This expansion means that planets in the pre-MS HZ are located further from their host stars, making them more resolvable for current and future astronomical observations.

The paper highlights that planets around cooler stars might maintain habitable conditions for up to 2.5 billion years post-accretion, due to higher stellar luminosities and the resultant larger planet-star separations during the pre-MS phase. However, this advantage is counteracted by the potential for significant water loss. For stars cooler than K5, including M stars, planets initially in the MS HZ receive fluxes exceeding the runaway greenhouse threshold, suggesting substantial initial water loss unless replenishment occurs.

Implications and Theoretical Considerations

The results carry significant implications for both observational astronomy and theoretical models of planetary formation and habitability. This research indicates that M-star planets, in particular, would require an ample initial endowment of water or subsequent delivery processes to retain their habitability post the pre-MS phase. The notion that these planets need to accrete more water than Earth to maintain habitability lends credence to alternate planetary formation theories, such as the in-situ accretion model which posits that sufficient volatile acquisition is feasible.

Furthermore, the paper directly addresses the potential for water loss through energy-limited hydrogen escape mechanisms, providing detailed formulations and results across the spectrum of star types. It identifies a stark contrast in potential ocean losses among spectral classes, with M8 stars anticipated to experience the most significant depletion.

Observational Prospects and Future Directions

The implications for observational astronomy are also marked; pre-MS HZs are accessible targets for next-generation telescopes, such as TMT, GMT, and the E-ELT. These instruments could effectively resolve planetary systems around cool stars due to the extended pre-MS phase and larger planet-star separations.

In the future, this paper suggests expanding on modeling efforts to refine boundary conditions for HZ calculations and further understand the dynamical processes involved in volatile retention and loss. Enhancements in understanding H2O and H2 escape dynamics and their parameterization in varying stellar environments could also pave the way for a more nuanced understanding of exoplanet habitability.

Concluding Remarks

This paper contributes a pivotal understanding of the habitability prospects of planets during the formative pre-MS phase, opening pathways for observational pursuits and theoretical advancements. The major conclusion underlines the necessity for extensive water endowments for planets orbiting cooler stars—a consideration central to the ongoing exploration of habitable worlds in the cosmos.