NASA's Habitable Worlds Observatory and the Search for Life Beyond the Solar System

The conversation opens with the Habitable Worlds Observatory, which Tyson describes as NASA's next big flagship and a successor to Hubble, the James Webb Space Telescope, and the Roman telescope. Tyson notes that the name refers to only part of the mission's science scope.

Tyson and Kirshen then remark on how little of the galaxy has been surveyed, with Kirshen recalling a map of the Milky Way that marks about 6,000 discovered exoplanets and noting that zooming out shows how little has been explored.

Shkolnik says that in her lifetime the count went from knowing of zero exoplanets with certainty to more than 6,000 confirmed. By comparing that known sample with the stars that have not yet been searched or detected, she says it is reasonable to conclude that on average every star has at least one planet.

Tyson and Shkolnik then clarify the difference between not knowing and assuming. Shkolnik explains that scientists had no data showing stars had planets, which is different from presuming they had none.

Tyson says he prefers the term 'Goldilocks zone' to 'habitable,' and Shkolnik responds that the Goldilocks zone is a simplified idea, describing it as a ring around a star where the temperature is right. Liquid water also requires an atmosphere to exist, so a planet in that zone needs more than the right temperature.

Shkolnik explains that a planet with water, an atmosphere, and a well-behaved star has the potential to be habitable. She states that this does not mean the planet is inhabited, which she calls a completely different question.

Tyson asks how biased the search is toward Earth-like life, noting that life on Earth has been found in extreme environments such as volcanic vents, which get energy from other sources and can be extremely hot, acidic, or alkaline.

Shkolnik says that if the search is limited to life as we know it, life could be very similar to ours or very different, so looking for what we know is still critical. She adds that astronomy has repeatedly shown that the Earth and the Sun are not special, and that the same reasoning suggests life on other planets may be chemically similar to ours.

Shkolnik explains that HWO is a single telescope, one observatory with multiple instruments. Each instrument can work at different wavelengths and resolutions, but all of them are fed by one mirror, which she notes is technically a series of mirrors.

Tyson and Shkolnik then discuss the timeline. HWO has not yet been built, and it will be pursued as quickly as possible. Shkolnik describes how decadal surveys set priorities every ten years and how the project's total cost remains unclear until its instruments and international partners are determined.

Shkolnik says the team hopes to make HWO robotically serviceable and modular, so that a robot could remove an older instrument and plug in a new one. She notes that this would make HWO the first modular telescope at the second Sun-Earth Lagrange point, L2, and that Hubble was serviced by astronauts.

The main mirror is expected to be between six and eight meters across. Shkolnik says the size is driven by the science, and that the launch vehicle will be a limiting factor, along with the need to fold the telescope to fit inside the fairing, as the James Webb Space Telescope was folded for its 2021 launch.

Asked whether HWO will target known planets or have its own search mode, Shkolnik says that for its main science goal of finding Earth-like planets around Sun-like stars and looking for life on them, the telescope will search for planets. The team will not necessarily know in advance that the planets exist, so HWO will find them and then characterize them.

Shkolnik describes HWO as both a finder scope and a life finder. Tyson remarks that the telescope creates its own data set, which researchers then investigate.

Shkolnik says that SPARCS, as a single small space telescope, does not need to go through the decadal survey process because it is far too small. She gives its total cost as about $10 million, while the total cost of HWO is a few billion dollars. She says small satellites punch above their weight.

SPARCS studies ultraviolet light from stars that host exoplanets so that researchers can understand the context in which biosignatures will be evaluated. Shkolnik explains that UV light can damage an atmosphere and therefore liquid water if there is enough of it, and can also chemically modify an atmosphere so that signs of life are either falsely detected or hidden, which is why the UV must be understood.

Shkolnik explains that low-mass stars have highly turbulent atmospheres and stronger magnetic fields at their surfaces than the Sun. They can produce more and more energetic flares and potentially more coronal mass ejections. She notes that researchers expect coronal mass ejections on these stars but have not confirmed one on any of them before.

Because a star is a plasma with free electrons locked to its magnetic field, Shkolnik says material moves with the field, so when the field pops off the surface it carries material with it.

Shkolnik notes that low-mass M stars dominate the galaxy, so their planets dominate the planet population. She says that although HWO has not existed for two decades, she has been thinking about habitable planets throughout her career, guided by one central question: what is the distribution of habitable environments in the galaxy.

Answering that question requires understanding planets around low-mass stars, which she calls a huge part of it. She also says that the scope is broad enough to include moons, and that HWO focuses on surface life rather than subsurface life, which she states is not part of HWO's goals.

Shkolnik says SPARCS has launched and is already operating. In the future, she says the team wants to combine hundreds of small satellites, possibly in the radio wavelength range, into a concept called the Mega Wave radio surveyor. One of its driving science cases might be looking for Earth-sized magnetic fields around planets in habitable zones.

She explains that because radio wavelengths are much longer than ultraviolet, a detector would need to be large, and that many small satellites working together would act like one observatory, which she describes as faking a large telescope relatively cheaply. Shkolnik compares this to the Very Large Array in New Mexico.

Tyson notes that about 6,000 exoplanets cluster near our spot in the Milky Way, and that most were found by Kepler and other space observatories, none of them as large as HWO. Shkolnik says HWO's search for life will keep it relatively close, because the rocky planets it targets in the habitable zones of Sun-like stars are very faint.

Shkolnik explains that the closest planets are the brightest, and that a planet must be far enough from its star to be separated on the sky. She puts the practical limit at roughly 50 parsecs, which Tyson notes is about 150 light-years.

Shkolnik says SPARCS's primary mission is studying low-mass stars and understanding UV effects on their planets. After that, the team will move to HWO's targets, which are much brighter than low-mass stars and therefore easier for SPARCS to observe. This will provide a baseline ultraviolet characterization of any planets HWO discovers.

Tyson then asks about the James Webb Space Telescope, which was designed as an infrared telescope to capture ultraviolet light from the early universe, which has been redshifted by cosmic expansion into the infrared.

Shkolnik says Hubble, partly named after Edwin Hubble, had as one of its primary objectives establishing the universe's expansion rate, which was uncertain by a factor of two at the time. Because the expansion rate relates to the universe's age, the estimate ranged between 10 billion and 20 billion years.

Hubble's science goals were prioritized, and much of its time went to measuring the Hubble constant. That work narrowed the age to about 14 billion years, plus or minus one, and the figure has been sharpened since. Shkolnik says the earlier range was correct but vague.

Tyson recalls that in graduate school the expansion rate of the universe was the question everyone wanted answered. Once it was measured, researchers stood at a new precipice and could ask questions they could not previously formulate, such as whether there is more than one universe.

Tyson says he now asks whether there is life on around 6,000 exoplanets orbiting the galaxy, and that he cannot predict the questions researchers will be asking ten years from now. He cites his saying that as the area of knowledge grows, so does the perimeter of ignorance.