The Universe is an endlessly fascinating place, and the prospect of alien life has captured the public’s imagination for centuries. However, believe it or not, the hunt for alien life is today a legitimate scientific endeavour. And in the last few decades, advancements in technology and scientific understanding make the search a realistic possibility.
Today, there are three main strategies scientists use to look for alien life:
- We’re exploring worlds within our own Solar System—such as Mars, Venus, and even Europa (one of Jupiter’s moons). The plan is to search remotely with fly-by missions, orbiters, and rovers. The aim is to look for evidence of past or present simple life, such as bacteria.
- We’re scanning the skies for artificial signals (primarily electromagnetic in nature) that would reveal the presence of technologically advanced extraterrestrial civilisations.
- We’re examining exoplanets—or planets that orbit stars outside our own Solar System—to find evidence of organic life on them.
All three strategies have advantages and disadvantages, but most scientists believe the last strategy is most likely to deliver our first success. Many scientists believe life requires conditions similar to those found on Earth. If true, then Earth is likely the only world in our Solar System where life ever existed; therefore, the first strategy is unlikely to succeed.
Furthermore, finding success in searching for artificial alien signals hinges on the assumption that technologically advanced civilisations are broadcasting their existence near Earth. Many scientists find this assumption naively optimistic, meaning the second strategy is also unlikely to succeed.
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Even if a small fraction of exoplanets with Earth-like properties have life on them (which is a fair assumption), this means finding life through exoplanet research has a much higher chance of success than the first two strategies.
Exoplanet research has come a long way since our first discovery of an exoplanet in 1992. Today, we’ve confirmed over 5,000 exoplanets in our Milky Way Galaxy, and our progress is thanks to our ability to make bigger and better telescopes.
And although modern-day telescopes are marvels of science and engineering, the ones we currently operate can only take us so far. To determine if life exists on an exoplanet, we’ll need to directly image Earth-like exoplanets, which have sizes and temperatures similar to Earth and orbit Sun-like stars.
The Habitable Worlds Observatory
Those capabilities are precisely what NASA is aiming for with its newly announced flagship mission: the Habitable Worlds Observatory (HWO).
To judge if an exoplanet is inhabited, there are some things we need to know, such as:
- Does the exoplanet have an atmosphere? If so, do the gases in it suggests biological activity?
- Does it have clouds, precipitation, and weather cycles?
- Does the planet show seasonal variations, like Earth has?
Unfortunately, we need better technology to better examine the properties of Earth-like exoplanets. The James Webb Space Telescope (JWST) and the next-generation Nancy Grace Roman Space Telescope are big steps in the right direction. Still, an improved observatory with better capabilities is required to image a truly Earth-like planet.
With the HWO, NASA is following through on a goal from astronomy’s decadal survey—dubbed Astro2020. Astro2020 is a report meant to serve as a community-led wish list guiding funding agencies and lawmakers. Published in November 2021, the report called for a US$11 billion, 6-metre telescope able to detect ultraviolet, visible, and near-infrared light.
The report specified that the telescope must be capable of detecting signs of life on 25 nearby Earth-like exoplanets—the minimum needed to confirm statistically whether life is commonplace in our Milky Way Galaxy. In Astro2020, there were two proposals made regarding telescopes for exoplanet research:
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- HabEx: a single-mirror telescope designed to directly image the Earth-like exoplanets near us
- LUVOIR: a giant segmented telescope meant to be an all-purpose “dream” observatory
It turns out the Habitable Worlds Observatory will be a scaled-up version of the HabEx, taking features from both the HabEx and the LUVOIR.
The Technology Behind the HWO
The specifications proposed so far are very encouraging. Firstly, the HWO will likely employ a segmented optical mirror design similar to what the JWST is currently using. Leveraging this design allows the telescope to focus the light into the compact regions of its detectors.
Furthermore, the HWO will use the same type of coronagraph technology currently being developed and tested for the Nancy Grace Roman Space Telescope. It can be challenging to detect and study exoplanets because the host star can be much brighter than the orbiting exoplanet. A coronagraph blocks out the star’s light, allowing the exoplanet’s faint light to be detected and analysed.
The primary mirror of NASA’s James Webb Space Telescope consisting of 18 hexagonal mirrors looks like a giant puzzle piece standing in the massive clean room of NASA’s Goddard Space Flight Center in Greenbelt, Maryland – Image Credit: NASA.
Scientists plan to have the HWO observe from the L2 Lagrange point, a point in space located approximately 1.5 million kilometres from Earth. Lagrange points are special positions where the gravitational forces of two large bodies (in this case, the Earth and the Sun) balance the centrifugal force felt by a smaller object (such as a spacecraft). This location makes it easy for a spacecraft to remain in a stable position and keep pace with the Earth’s orbit around the Sun.
The design of the HWO will be a game of tradeoffs between how many Earth-like candidates we think it can directly image versus how much it will cost. The challenge here is that searching for life beyond Earth involves many unknowns. Each Earth-like planet we directly image and characterise might be inhabited, but we have no idea how probable that is. We’ll only better understand the unknowns after the findings from HWO come in.
Though, the HWO’s first, and likely its most difficult, obstacle will be convincing the US Congress to fund it at all. In 2023, Congress allocated US$1.51 billion to NASA’s astrophysics division; that sounds like a lot of money (and it is), but it’s actually a 4% decline in funding from the previous year.
However, one big advantage of the HWO is our ability to fix problems and update the telescope after launch. This means we don’t need the funding for all the bells and whistles at launch; they can be added to the telescope later when there’s funding for it. Furthermore, ongoing repairs and upgrades mean that the telescope can stay operational for longer. From Congress’ perspective, this may make the HWO a better return on investment and perhaps more worthy of funding.