Alien planets: A first-of-its-kind observatory will boost the search for exoplanets

It contains a huge starshade that blocks light from stars to help giant telescopes study distant exoplanets.
Deena Theresa
An exoplanet in outer space.Marje/iStock

Recently, NASA confirmed the discovery of more than 5,000 exoplanets, raising the possibility of alien life on other worlds.

But getting an accurate picture of these exoplanets, let alone determining if they are habitable, is immensely complex. 

"The ability to look for exoplanets - planets that orbit around stars - is one of the hardest optical engineering challenges we've ever tried. And it's not been solved," John C Mather, senior astrophysicist at the NASA Goddard Space Flight Center, told IE. "It's a matter of high priority for astronomers because we would definitely like to know if we're alone."

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All the light that we don't want to see

I know what you're thinking: Why can't we point a gigantic telescope at a star out there and look for its planets?

It isn't that simple.

The light reflected off a planet is around 10 billion times fainter than a star. This acts as a major obstacle, as you're trying to look for a faint planet that's orbiting an extremely bright star.

But surely there has to be a solution?

NASA has a few on deck, starting with the James Webb Space Telescope, that will capture the intensity of light emitted by a given section of the atmospheres of exoplanets, one of its many missions. The Webb telescope will make it possible to measure the three-dimensional variation in the atmospheres of exoplanets, gaining valuable knowledge.

Another method is to use a high-contrast coronagraph - an instrument that suppresses starlight from inside a telescope. This will be a feature of the Nancy Grace Roman Space Telescope, which is expected to launch in 2027.

The technology employed - starlight suppression - is crucial. It involves suppressing light from the associated star, giving scientists a chance to collect the tiniest amount of reflected light from the exoplanet.

But the concept doesn't have to be confined to a telescope.

A simple system

Alien planets: A first-of-its-kind observatory will boost the search for exoplanets
An artist's depiction of a sunflower-shaped starshade that could help space telescopes find and characterize alien planets. Source: NASA/JPL/Caltech

A gigantic, flower-like object known as a starshade works similarly to a coronagraph, except it is located outside a telescope.

The starshade, which resembles a sunflower, could propel itself into a position that is miles ahead of a space telescope. The flower-shaped petals of the starshade play a key role in its effectiveness.

The shape of the petals creates a softer edge, which causes less bending of light waves. This would cast a dark shadow on the telescope, blocking the light from the host star and allowing the space telescope to capture light directly from planets in the star's orbit.

The telescope would be able to take images of the planets without being blinded by starlight.

Mather says that the idea isn't new. 

"In the 2000s or so people were considering the idea of a starshade that could work with the James Webb Space Telescope. At the time, we didn't know how to do it, and it seemed too hard. So we did not try. Then in 2007, a solution was found for the shape of the starshade so that it could work better. And then we tried to propose it to work with the Webb telescope again, but that proposal also failed," Mather tells us.

In the back of his mind, he wondered. Does it have to have a telescope in space? What if the telescope itself could be on the ground? But it sounded complex, so he set the idea aside.

A hybrid observatory in the making

Four years ago, Mather went back to the idea. "The Europeans are building the biggest telescope ever imagined on the ground in South America. It's 39 meters across, which is six times as big as the Webb telescope. So if you could use it for wonderful purposes, like looking for exoplanets, that would be a fantastic accomplishment. We're just beginning in this process of finding if life could exist, and I thought this was one possible way to go at it," he said.

So, a few months ago, Mather wrote a short proposal that stated that, though people have been working on starshades for a long time, they are not very practical when they are required to be enormous - as we needed them to work with a telescope on the ground. 

"To work with the telescope on the ground, which is 39 meters across, we need an even bigger starshade, which is about 100 meters across. How do we get that into space? We have a concept of just extrapolating from smaller designs. And we tried that, but it would not quite fit in the rocket that we could get at that time," said Mather.

So, he tried something else.

"Let's start at the beginning and assume there is an alternate way to construct it. If we could get the mass of this object down to a much smaller value, then the cost would be smaller, the amount of fuel that it takes to push it around in space would be smaller, and everything would be better," he explained.

Based on his ideas, Mather proposed the first hybrid observatory, combining a 100m diameter starshade with a telescope on the ground. The Hybrid Observatory for Earth-like Exoplanets (HOEE) would convert the largest ground-based telescopes now under construction (Giant Magellan Telescope, Thirty Meter Telescope, and Extremely Large Telescope) into the most powerful planet finders yet designed. 

To further develop his concept - the most powerful exoplanet observatory yet proposed - Mather was among the 12 researchers who received Phase I grants in 2022 from the NASA Innovative Advanced Concepts (NIAC) program.

Addressing  the highest priority recommendation of the Exoplanet Strategy report

The "Pathways to Discovery in Astronomy and Astrophysics for the 2020s", also known as the decadal survey, which was released in November 2021, mentioned the finding and characterizing of Earth-like exoplanets that could harbor life as among three priority science areas.

"What the decadal survey from the National Academy of Sciences said was that we need to see these little planets. And they thought the best tool would be a six-meter diameter Space Telescope, which would be much more accurate and stable than the Webb telescope. We know we can build something that big now, so that's the preferred solution," said Mather.

The advantages are many. But it also contains a set of exciting challenges.

"Nobody has ever built the kind of perfect optics that it would require. So we're working on that, and we are probably going to succeed. But suppose that's too way is to build a starshade. So it casts a shadow onto the telescope. But the telescopes we can get in space are smaller than what we can build on the ground. So what I'm hoping for is that there's a way to use the big telescopes on the ground for contributing to this scientific process," he explained.

What are the advantages of using a larger telescope on the ground?

"Firstly, it is much more sensitive. So, it can see smaller planets more quickly. More stars are potential targets," he said.

"But the difficulty that we face is that the observatory is moving as the Earth rotates. And so the [starshade] in space has to keep up with that. Otherwise, the alignment between the star target and the telescope on the ground will only last for a moment," he said.

Making a mark in exoplanet science

Alien planets: A first-of-its-kind observatory will boost the search for exoplanets
Graphic depiction of Hybrid Observatory for Earth-like Exoplanets (HOEE). Source: John Mather

The HOEE depends on two major innovations: a ground-space hybrid observatory and an enormous telescope on the ground. 

In an effectively geostationary orbit, a starshade would need to match the position and velocity of the moving telescope and cast a dark shadow that blocked light from the star without blocking the light of its planets. According to HOEE's abstract, while active propulsion would maintain alignment during the observation, adaptive optics in the telescope would make up for atmospheric distortion of the incoming images.

The HOEE would observe reflected light, influenced by surface minerals, oceans, continents, weather, and atmospheric constituents of the planets, with low-resolution spectroscopy. This would mean it can observe the presence of photosynthetic oxygen, which could indicate the presence of life and its prevalence. 

The starshade would operate as a separate spacecraft.

"In our case, to work with a telescope on the ground, it would have to be 170,000 kilometers away in space but still orbiting around the Earth. For telescopes that are already in space, it doesn't have to be that far away," said Mather.

Despite the distance, it's not that hard to tell if the starshade is in the right place. "If it isn't lined up correctly, the stars will start to shine on the telescope, and the shadow won't be good enough anymore. The hard problem is pushing the starshade - a lot of rocket fuel is required to push the starshade into the right place for more than a moment," said Mather.

This is why it is imperative to find a lower weight and mass designed for the starshade.

"The one that we designed would have required about 7,000 kilos, which seems an awful lot because what we need is just something to cast a shadow. It doesn't have to be heavy. It's only got to be heavy because it has to withstand being launched into space and deploy itself some way," he says.

But maybe there's another approach.

"So that's our question - is there another way to make something much lighter and much cheaper so that this would become a more attractive scientific capability? Low mass is important for observing many different target stars," he said.

A very exciting task ahead

Could the starshades be more efficient than high-contrast coronagraphs?

"It depends on how efficient those coronagraphs can be. If you can build the perfect optics that you'd like to have, then they're great. And you can point your telescope, and then you start anytime, which is perfect," said Mather.

That would have a massive advantage as one can look anywhere.

"But the trouble with it is the telescope as even a six-meter telescope, as big as the Webb Telescope. is still not very big compared to the problem that we're trying to solve. So it might take you weeks to observe a particularly interesting star to see if it has planets. And that's a long time. It's not that you can't do it. But if you could do in an hour what it would take a week with a space telescope, then a grand telescope could have an advantage," explained Mather.

That is one of the exciting things about having a big telescope on the ground. "It is so much faster, maybe 1,000 times faster than the six-meter telescope in space," said Mather.

But how far are we from the HOEE?

"It's hard to tell. If we were lucky and found a design that everyone liked, you could ask a big aerospace company to build one of these starshades. And they would probably be able to do it in 10 years. But I don't think it's going to be easy. So ideally, the great observatory that the National Academy recommended would fly around 2045, which is too long after the Webb telescope might complete its fuel. So we have a while to be able to think about this," said Mather. "It's very difficult. That's why it was selected by the NASA Institute of Advanced Concepts. Which means honestly, we don't really know how to solve this problem yet."

The HOEE is brilliant and futuristic - and could take decades to come to fruition. But once it does, it could be a gamechanger for exoplanet science.