To get the most effective direct photographs of exoplanets with area telescopes, we’ll search for star shadows

The James Webb (JWST) and Nancy Grace Roman (RST) space telescopes will be launched into space between 2021 and 2024. Successor to several observatories (such as Hubble, Kepler, Spitzer, and others), these missions will carry out some of the most ambitious astronomical surveys ever undertaken. This ranges from discovering and characterizing extrasolar planets to researching the secrets of dark matter and dark energy.

In addition to advanced imaging functions and high sensitivity, both instruments also carry coronagraphs – instruments that suppress darkening starlight so that exoplanets can be recognized and observed directly. According to a selection of articles recently published by the Journal of Astronomical Telescopes, Instruments, and Systems (JATIS), we will need more of these instruments if we are to really study exoplanets in detail.

This special section on Starshades contains publications (published between January and June 2021) that cover the latest scientific, technical, research, and programmatic advances made with coronographs. These instruments, also known as Starshades, address one of the greatest challenges in the identification and characterization of exoplanets. In summary, it can be said that the vast majority of known exoplanets (4,422 confirmed to date) were discovered indirectly.

Exoplanet studies

Of these methods, the most widespread and most effective are the transit method (transit photometry) and the radial velocity method (Doppler spectroscopy). In the former, astronomers monitor stars for periodic decreases in brightness, which are a possible indication that an exoplanet (or more) is in orbit in front of the parent star (also known as a transit) relative to the observer.

In the latter, astronomers measure how a star moves back and forth (and how fast) to measure the gravitational influence of satellites in orbit. Regardless of this, these methods are also effective in determining the radius (transit) of an exoplanet and its mass (radial velocity). Together, they are the most effective means of confirming and characterizing exoplanets and limiting their potential habitability.

On rare occasions, astronomers have been able to observe exoplanets directly by detecting starlight reflected from their atmosphere – also known as. the direct imaging process. Unfortunately, most of these planets were gas giants or had long orbits around their star (or both). For smaller, rocky planets with shorter orbits (where more Earth-like planets are found), the light reflected from their atmosphere is likely to be washed out by their sun.

For this reason, coronagraphs have been the subject of significant research and development in recent years. In addition to instruments that can be integrated into observatories, NASA also intends to build a spacecraft that can work in conjunction with space telescopes to suppress obscuring starlight. These efforts are part of NASA’s Starshade Project, which is to provide a spacecraft with an extendable flower-shaped light shield.

The special part

Despite the advances made in the development of the Starshade (and related technology) in recent years, the news about those advances tends to be scattered. For this reason, the editors of the special section – all members of NASA’s Starshade Technology and Science working group – have collected 19 research papers representing the latest research (January to June 2021).

As the editors state in the introductory paper entitled “Special Section on Starshades: Overview and a Dialogue”:

“The star shadow is a technology that has undergone rapid development and great interest from many institutions. Much of the advances in this area are spread across many journals and conferences. It is therefore difficult to collect them in a single place to get a good overview of the state of the star shadows. “

“As interest in developing Star Shade-based missions grows, we hope this particular section serves as a tutorial and provides enough background information to potential investigators unfamiliar with Star Shade to get an up-to-date overview of the field in one location receive.”

This work is divided into six categories corresponding to four different research areas, all of which are presented in the second part – Summary and contributions – the special part. It starts with an overview of the Starshade program and the types of missions it will enable, followed by a series of technology related papers that consider the challenges of deployments and rendezvous, followed by a series of academic and mission related papers.

Summary and contributions

Many of the publications deal with the particular challenges Starshade faces in connection with a space telescope. For example part two – Operation and formation flight – presents papers examining the technical challenges of sending a mission into space in stowed formation and then dispatching it once it has reached its destination – similar to what James Webb will do after its launch (currently scheduled for November 2021).

Staying in formation with a space telescope is also a major challenge, especially when the telescope is moving from one target to another. Here, the papers presented consider various guidance, navigation, and propulsion systems and determine that a chemical propulsion system that does not require ground tracking and laser beacons would be the optimal arrangement.

The importance of proper planning is also addressed, which is as true for Starshade as it is for the proposed Remote Occulter – a wraparound Starshade concept that was developed for working with ground-based telescopes. In sections three and four, Starlight suppression and power modeling and Sunshine, another major challenge is addressed, namely the possibility of interference from zodiacal light and sunlight reflecting off Starshade’s petals.

Exploring the possibilities

Equally important are publications exploring the benefits of a Starshade mission in combination with next-generation telescopes such as the JWST and RST, as well as the Habitable Exoplanet Observatory (HabEx) and the Large UV / Optical / IR Surveyor (LUVOIR). These proposed missions will be optimized for direct imaging and characterization of exoplanet atmospheres and will likely be combined with a Starshade concept (or have their own coronograph instruments).

The many papers that the Star shadow program and NASA’s efforts to bring the Starshade to Technology Readiness Level 5 (TLR 5) – an effort overseen by the S5 project. This part also describes the efforts of the Starshade Exoplanet Data Challenge and the Roman Exoplanet Imaging Data Challenge (EIDC), both of which aim to combine the scientific requirements of future Starshade missions with specific performance parameters.

In the first case, researchers at NASA JPL relied on synthetic images of exoplanet systems generated by the Starshade Imaging Simulation Toolkit for Exoplanet Reconnaissance (SISTER). This versatile tool, managed by Caltech, is designed to provide accurate models of what exoplanet systems would look like if observed through a star’s shadow.

The EIDC, on the other hand, is a community effort led by the Turnbull CGI Science Investigation Team, which in turn is led by Margaret C. Turnbull – the renowned astrophysicist and researcher Margaret C. Turnbull of the SETI Institute. Launched in 2019, the Roman EIDC basically simulated what the Nancy Grace Mission Telescope will see with (and without) the help of a Starshade Rendezvous mission.

This is taken up later in sections five and six – Exoplanet detection and Observations – where research papers re-discuss the types of exoplanet systems that future telescopes will see with the help of Starshade. In some of these publications, sample images were provided and specific targets – nearby star systems that are considered optimal candidates – were discussed.

The Observations section concludes with three articles written by different teams, all led by lead researcher Eliad Peretz of NASA Goddard. Here, the research teams investigated how effectively space telescopes with Starshade would recognize and characterize potentially habitable exoplanets (based on different observation conditions) and how ground-based observatories would benefit from the remote occulter.

Artistic concept of the NASA HabEx space telescope in combination with the Starshade. Source: Gaudí et al.

Promote dialogue

The special part closes things off with a little tutorial called. from Dialogue about star shadows. This section raises and covers questions in the form of a dialogue between a hypothetical student (named Morgan Nemandi) and a Starshade “expert” (Urania Sage). The questions were based on actual questions asked by members of the amateur astronomy and space exploration enthusiast communities.

The dialogue also pays homage to Galileo’s Dialogue Concerning the Two Chief World Systems (It: Dialogo sopra i due massimi sistemi del mondo), a treatise he published in 1632 that presented arguments for the heliocentric model of the universe of Copernicus. It was also presented in a format that emphasized how many of the questions people have about space exploration are things they don’t always like to ask (i.e., “things I’m afraid to ask”).

This last part of the special is organized in a four day format that reflects the original dialogue. While Morgan and Urania cover the basics of Starshade and the history of the project on the first day, they cover engineering and technology questions on the second day, science questions on the third day, and program-related questions on the fourth day.

While the Starshade development schedule has not yet been released, it is clear that NASA and other space agencies intend to pursue this technology. In the years to come, starlight suppressors and coronographs are likely to become an integral part of next-generation astronomy and exoplanet research. In combination with the James Webb, Roman, TESS, HabEx and LUVOIR space telescopes, the number of known rocky exoplanets will increase exponentially.

Starshade technology will also support missions such as ESA’s proposed characterizing ExOPlanets Satellite (CHEOPS), PLAnetary Transits and Oscillations of star (PLATO) and Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) space telescopes. These missions will also serve to complete the census of terrestrial exoplanets, characterize their atmospheres, and find evidence of life outside of our solar system.

Further reading: SPIE Digital Library, NASA Exoplanet Program

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