The Habitable Exoplanet Observatory (HabEx) is a

space telescope A space telescope (also known as space observatory) is a telescope in outer space used to observe astronomical objects. Suggested by Lyman Spitzer in 1946, the first operational telescopes were the American Orbiting Astronomical Observatory, OAO ...

concept that would be optimized to search for and image Earth-size habitable exoplanets in the habitable zones of their stars, where

liquid water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known living organisms ( ...

can exist. HabEx would aim to understand how common terrestrial worlds beyond the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

may be and determine the range of their characteristics. It would be an optical, UV and

infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...

telescope that would also use spectrographs to study planetary atmospheres and eclipse

starlight Starlight is the light emitted by stars. It typically refers to visible electromagnetic radiation from stars other than the Sun, observable from Earth at night, although a component of starlight is observable from Earth during daytime. Sunlig ...

with either an internal

coronagraph A coronagraph is a telescopic attachment designed to block out the direct light from a star or other bright object so that nearby objects – which otherwise would be hidden in the object's bright glare – can be resolved. Most coronagr ...

or an external starshade. The proposal, first made in 2016, is for a

large strategic science missions NASA's large strategic science missions or large strategic missions, formerly known as Flagship missions or Flagship-class missions, are the costliest and most capable NASA science spacecraft. Flagship missions exist within all four divisions ...

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

mission. It would operate at the Lagrange point L2. In January 2023, a new space telescope concept was proposed called the

Habitable Worlds Observatory The Habitable Worlds Observatory (HWO) is a proposed next generation space telescope, a successor of the flagship Hubble, Webb and Roman projects. It would have a large 6–8 meter mirror and be able to detect infrared, optical, and ultraviolet ...

(HWO), which draws upon HabEx and the

Large Ultraviolet Optical Infrared Surveyor The Large Ultraviolet Optical Infrared Surveyor, commonly known as LUVOIR (), is a multi-wavelength space telescope concept being developed by NASA under the leadership of a Science and Technology Definition Team. It is one of four large astro ...

(LUVOIR).

Overview

In 2016, NASA began considering four different

s as the next Flagship (

Large strategic science missions NASA's large strategic science missions or large strategic missions, formerly known as Flagship missions or Flagship-class missions, are the costliest and most capable NASA science spacecraft. Flagship missions exist within all four divisions ...

) following the

James Webb Space Telescope The James Webb Space Telescope (JWST) is a space telescope designed to conduct infrared astronomy. As the largest telescope in space, it is equipped with high-resolution and high-sensitivity instruments, allowing it to view objects too old, Lis ...

and

Nancy Grace Roman Space Telescope The Nancy Grace Roman Space Telescope (shortened as the Roman Space Telescope, Roman, or RST) is a NASA infrared space telescope in development and scheduled to launch to a Sun–Earth L2 orbit by May 2027. It is named after former NASA Chie ...

. They are the Habitable Exoplanet Observatory (HabEx),

(LUVOIR),

Origins Space Telescope Origins Space Telescope (''Origins'') is a concept study for a far-infrared survey space telescope mission.Lynx X-ray Surveyor. In 2019, the four teams turned their final reports over to the

National Academy of Sciences The National Academy of Sciences (NAS) is a United States nonprofit, NGO, non-governmental organization. NAS is part of the National Academies of Sciences, Engineering, and Medicine, along with the National Academy of Engineering (NAE) and the ...

, whose independent

Decadal Survey A decadal survey is a 10-year plan outlining scientific missions and goals created by the United States National Academies. It is a summary of input from scientists in the United States and beyond. Examples include: * Astronomy and Astrophysics ...

committee advises NASA on which mission should take top priority. The Habitable Exoplanet Imaging Mission (HabEx) is a concept for a mission to directly image planetary systems around Sun-like stars. HabEx will be sensitive to all types of planets; however its main goal is to directly image Earth-size rocky exoplanets, and characterize their atmospheric content. By measuring the spectra of these planets, HabEx will search for signatures of habitability such as water, and be sensitive to gases in the atmosphere potentially indicative of biological activity, such as oxygen or ozone. In 2021, the National Academy of Sciences released its final recommendations in the Decadal Survey. It recommended that NASA consider a new 6-meter (20-foot) aperture telescope combining design elements of LUVOIR and HabEx. The new telescope would be called the

(HWO). A preliminary launch date was set for 2040, and the budget was estimated to be $11 billion.

Science drivers and goals

HabEx's prime science goal is the discovery and characterization of Earth-sized planets in the habitable zones of nearby main sequence stars, it will also study the full range of exoplanets within the systems and also enable a wide range of general astrophysics science. In particular, the mission will be designed to search for signs of

habitability Habitability is the adequacy of an environment for human living. Where housing is concerned, there are generally local ordinances which define habitability. If a residence complies with those laws, it is said to be habitable. In extreme environ ...

and

biosignature A biosignature (sometimes called chemical fossil or molecular fossil) is any substance – such as an element, isotope, molecule, or phenomenon – that provides scientific evidence of past or present life on a planet. Measurable ...

s in the atmospheres of Earth-sized rocky planets located in the

habitable zone In astronomy and astrobiology, the habitable zone (HZ), or more precisely the circumstellar habitable zone (CHZ), is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressu ...

of nearby solar type stars. Absorption features from , , , and CO, and emission features from Na and K, are all within the wavelength range of anticipated HabEx observations. With a contrast that is 1000 times better than that achievable with the

Hubble Space Telescope The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...

, HabEx could resolve large dust structures, tracing the gravitational effect of planets. By imaging several faint

protoplanetary disk A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may not be considered an accretion disk; while the two are sim ...

s for the first time, HabEx will enable comparative studies of dust inventory and properties across a broad range of

stellar classification In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction gratin ...

s. This will put the

in perspective not only in terms of exoplanet populations, but also in terms of dust belt morphologies.

General astronomy

General

astrometry Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other Astronomical object, celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, th ...

and

astrophysics Astrophysics is a science that employs the methods and principles of physics and chemistry in the study of astronomical objects and phenomena. As one of the founders of the discipline, James Keeler, said, astrophysics "seeks to ascertain the ...

observations may be performed if justified by a high science return while still being compatible with top exoplanet science goals and preferred architecture. A wide variety of investigations are currently being considered for HabEx general astrophysics program. They range from studies of galaxy leakiness and inter-galactic medium

reionization In the fields of Big Bang theory and physical cosmology, cosmology, reionization is the process that caused electrically neutral atoms in the primordial universe to reionize after the lapse of the "Timeline of the Big Bang#Dark Ages, dark ages". ...

through measurements of the escape fraction of ionizing photons, to studies of the life cycle of

baryon In particle physics, a baryon is a type of composite particle, composite subatomic particle that contains an odd number of valence quarks, conventionally three. proton, Protons and neutron, neutrons are examples of baryons; because baryons are ...

s as they flow in and out of galaxies, to resolved stellar population studies, including the impact of massive stars and other local environment conditions on star formation rate and history. More exotic applications include astrometric observations of local

dwarf galaxies A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is so ...

to help constrain the nature of

dark matter In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravity, gravitational effects that cannot be explained by general relat ...

, and precision measurement of the local value of the

Hubble Constant Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther a galaxy is from the Earth, the faste ...

. The following table summarizes the possible investigations currently suggested for HabEx general astrophysics:

Preliminary desired specifications

Based on the science drivers and purpose, the researchers are considering direct imaging and

spectroscopy Spectroscopy is the field of study that measures and interprets electromagnetic spectra. In narrower contexts, spectroscopy is the precise study of color as generalized from visible light to all bands of the electromagnetic spectrum. Spectro ...

of reflected starlight in the

visible spectrum The visible spectrum is the spectral band, band of the electromagnetic spectrum that is visual perception, visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' (or simply light). The optica ...

, with potential extensions to the UV and the

near infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those o ...

parts of the

spectrum A spectrum (: spectra or spectrums) is a set of related ideas, objects, or properties whose features overlap such that they blend to form a continuum. The word ''spectrum'' was first used scientifically in optics to describe the rainbow of co ...

. The telescope has a primary monolithic mirror that is in diameter. An absolute minimum continuous wavelength range is 0.4 to 1 μm, with possible short wavelength extensions down below 0.3 μm and

extensions to 1.7 μm or even 2.5 μm, depending on the cost and complexity. For characterization of

extraterrestrial atmosphere The study of extraterrestrial atmospheres is an active field of research, both as an aspect of astronomy and to gain insight into Earth's atmosphere. In addition to Earth, many of the other astronomical objects in the Solar System have atmosphe ...

s, going to longer

wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...

s would require a starshade that would launch separately on a

Falcon Heavy Falcon Heavy is a super heavy-lift launch vehicle with partial reusability that can carry cargo into Earth orbit and beyond. It is designed, manufactured and launched by American aerospace company SpaceX. The rocket consists of a center core ...

, or a larger telescope in order to reduce the amount of background light. An alternative would be to keep the

small. Characterizing exoplanets at wavelengths shorter than ~350 nm would require a fully UV-sensitive high contrast optical train to preserve throughput, and will make all wavefront requirements more stringent, whether for a starshade or a coronagraph architecture. Such high spatial resolution, high contrast observations would also open up unique capabilities for studying the formation and evolution of stars and galaxies.

Biosignatures

HabEx would search for potential

gases in exoplanets' atmospheres, such as (0.69 and 0.76 μm) and its photolytic product

ozone Ozone () (or trioxygen) is an Inorganic compound, inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , break ...

(). On the long wavelength side, extending the observations to 1.7 μm would make it possible to search for strong additional water signatures (at 1.13 and 1.41 μm), and would also allow to search for evidence that the detected and gases were created by abiotic processes (e.g., by looking for features from , CO, ). A further infrared capability to ~2.5 μm would allow to search for secondary features such as methane ( ) that may be consistent with biological processes. Pushing even further in the UV may also allow distinction between a biotic, high-O₂ atmosphere from an abiotic, -rich atmosphere based on the ozone absorption of 0.3 μm.

Molecular oxygen There are several known allotropes of oxygen. The most familiar is molecular oxygen (), present at significant levels in Earth's atmosphere and also known as dioxygen or triplet oxygen. Another is the highly reactive ozone (). Others are: * Atomic ...

() can be produced by geophysical processes, as well as a byproduct of

photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...

life forms The following outline is provided as an overview of and topical guide to life forms: A life form (also spelled life-form or lifeform) is an entity that is living, such as plants (flora), animals (fauna), and fungi ( funga). It is estimated tha ...

, so although encouraging, is not a definite biosignature, unless it is considered in its environmental context. I.e., while O2 production to ~20% of atmospheric content seems to be part of life on Earth, too much oxygen is actually poisonous to life as humans know it and could easily be created by planetary situations like a incredibly deep world spanning ocean.

References

External links

Final report 2019 at JPL/NASA

HabEx documents at JPL/NASA
{{Astrobiology Space telescopes Infrared telescopes Exoplanet search projects Proposed astrobiology space missions Proposed NASA space probes