Want to find habitable planets outside the solar system? You have to ask the "Trisolarans" for territory

Produced by: Science Popularization China

Author: Wang Shanqin

Producer: China Science Expo

On January 9, 2023, a paper with Jacob Lustig-Yaeger and Guangwei Fu as co-first authors announced that the James Webb Space Telescope (hereinafter referred to as "Webb") confirmed a new planet outside the solar system (exoplanet) and obtained its transmission spectrum. [Note 1]

Because the mass and radius of this planet are close to those of the Earth, some media even said that Webb had discovered a habitable planet (that is, a planet suitable for the survival and evolution of life).

So how does Webb confirm the existence of an exoplanet? Is this planet really habitable? If one day we want to go to the "next Earth", where should our destination be?

Let us follow the scientists' research and find the answers to these questions step by step.

1. TESS discovers transit signals, inferring the existence of exoplanets

First of all, please note that in the above we said that Webb "confirmed" an exoplanet, not "discovered" it, because it was actually the Transiting Exoplanet Survey Satellite (TESS) that first discovered it.

TESS in the clean room

(Image credit: NASA)

According to the naming rules for exoplanets, the first confirmed planet around a star is numbered after the star number plus the letter b. The parent star of the exoplanet confirmed by Webb is LHS 475, so the number of this exoplanet is LHS 475b.

LHS 475 is a red dwarf star located in the direction of the constellation Antarctica, about 41 light-years away from Earth (1 light-year = 9.46 trillion kilometers).

An artist's impression of a red dwarf star and a planet orbiting it.

(Image source: National Science Foundation)

Red dwarfs belong to the main sequence stars. What are main sequence stars? Simply put, main sequence stars are stars that only undergo hydrogen fusion reactions inside. Our sun is also a main sequence star, and it belongs to a yellow dwarf. The "red" and "yellow" in red dwarfs and yellow dwarfs respectively reflect their approximate colors. The temperature of red dwarfs is lower than that of yellow dwarfs, so they appear red. Perhaps many readers are more familiar with white dwarfs than red dwarfs and yellow dwarfs. However, it should be noted that white dwarfs are not main sequence stars.

In theory, red dwarfs are the most common type of stars in the universe. Their mass, temperature, radius and thermal brightness are all lower than those of the sun. Previous studies have shown that LHS 475 has a mass of about 0.26 times that of the sun, a radius of about 0.28 times that of the sun, a surface temperature of about 3300 K (the surface temperature of the sun is 5772 K), and a thermal brightness of 0.009 times that of the sun.

LHS 475b was first inferred to exist because TESS observed 45**** periodic transit signals in the brightness of LHS 475.

What is a transit signal? This starts with one of the most important methods for searching for exoplanets: the “transit method”.

"Transit" refers to the dimming of a star's brightness caused by the light emitted by other much smaller celestial bodies being blocked. The "Venus transit" and "Mercury transit" phenomena caused by Venus and Mercury blocking the sun in the solar system are both transit phenomena.

Based on the periodic dimming of the brightness of stars, it can be inferred that they have undergone transit phenomena, and thus infer that they have planets.

Figure: Schematic diagram of a planet blocking part of the light emitted by a star, causing a transit phenomenon. The transit phenomenon causes a periodic depression in the star's brightness curve (light curve). Since the size of a planet is much smaller than that of a star, the brightness change caused by each occlusion is very weak.

(Photo credit: Hans Deeg)

TESS has four small telescopes with a diameter of 10 cm, and is equipped with four CCDs. The total field of view is up to 2,300 square degrees, which is equivalent to the area occupied by about 12,100 full moons in the sky, which is more than 5% of the entire sky area.

Therefore, it can observe a huge number of stars at one time.

In addition, TESS will observe each region continuously for at least 27 days. For planetary systems where transits occur every few days, it can observe several complete transits in one observation cycle.

When TESS's eyes are fixed on the region where LHS 475 is located, it sees LHS 475 transit for about 40 minutes at a time, with each dimming by 0.978 parts per thousand.

Such regular data means that it may be periodically obscured by a planet .

Therefore, this possible planet around LHS 475 was given a provisional designation, TOI 910.01. Subsequent studies have shown that if TOI 910.01 really exists, it orbits its parent star once every 2.029 days.

2. Webb takes over and confirms the identity of exoplanets

The team led by Lustig-Yaeger and Fu (hereinafter referred to as the "LF team") observed the brightness evolution of LHS 475 twice on August 31, 2022 and September 4, 2022 using Webb's Near-Infrared Spectrograph (NIRSpec), with each observation lasting 4.4 hours.

Near-infrared spectrometer in a clean room.

(Image source: Astrium GmbH - Astrium GmbH)

After obtaining the spectral data, the LF team synthesized the brightness of different bands into a total brightness.

The LF team's two observations confirmed that the brightness of LHS 475 decreases periodically, with each dimming ratio being 1.06 parts per thousand, which is very close to the 0.978 parts per thousand obtained from TESS data, which indirectly confirms the sensitivity of TESS.

The LF team's analysis of the data completely ruled out the possibility of a false positive, thus confirming that there is a planet periodically occulting the star : LHS 475 b (formerly TOI 910.01).

The brightness evolution diagram of the star LHS 475 obtained by Webb. The horizontal axis is time, in hours; the vertical axis is relative brightness. In order to make it easier to distinguish, the two sets of data obtained from the two observations were moved a certain distance along the vertical axis.

(Image source: Lustig-Yaeger & Fu, et al. 2023, arXiv:2301.04191v1)

This is the first exoplanet confirmed by Webb. Previously, although Webb had taken light curves and spectra of the exoplanet WASP-96b and images of HIP 65426b, they were all confirmed by other telescopes before Webb's observations.

Of course, before further research and confirmation, TESS's 45 transit observations are enough to convince most people that this exoplanet exists, because although phenomena such as sunspots and flares can cause slight changes in the overall brightness of a star, they cannot be so regular.

Artist's impression of LHS 475 shining upon LHS 475b.

(Image credit: NASA, ESA, CSA, Leah Hustak (STScI))

Based on the data, the LF team found that the semi-major axis of LHS 475b's orbit is about 15.87 times the radius of the star. Combined with the radius of the star, this value is about 0.02 AU (1 AU = the average distance between the Earth and the Sun = 149.6 million kilometers).

Such a close distance will cause a phenomenon called "tidal locking" to occur, causing one side of the planet to always face the star - so it is always daytime, and the other side to always face away from the star - so it is always nighttime.

3. Does LHS 475b have the potential to be a habitable planet? From temperature to atmosphere, three consecutive denials

Using previously obtained spectral data, the LF team calculated that the diameter of LHS 475b is about 99% of the Earth's diameter, and assuming that its density structure is similar to that of the Earth, they inferred that its mass is about 91% of the Earth's mass.

Artist's impression of LHS 475b.

(Image credit: NASA, ESA, CSA, L. Hustak (STScI))

At first glance, the planet appears to be about the same size as Earth, a similarity that could lead some to mistakenly believe LHS 475b is a habitable planet.

So, is it really livable?

In fact, there are many requirements for a planet to be habitable, the minimum requirement being that at least some areas are neither too cold nor too hot, allowing liquid water to exist.

However, remember? As mentioned earlier, LHS 475b is only 0.02 AU away from its parent star, so its overall average temperature is relatively high. Calculations by the LF team show that its overall average temperature reaches 586K (313 degrees Celsius); if it has no atmosphere or only a thin atmosphere, then the temperature of the side facing the star reaches 748K (475 degrees Celsius). Obviously, its temperature is too high and is not suitable for the existence of life.

In addition to the high temperature, the atmosphere of this planet may not be suitable for the survival and evolution of life.

Based on observations from Webb's near-infrared spectrometer, the LF team obtained the transmission spectrum of LHS 475, although the obtained transmission spectrum was of low accuracy and failed to show the characteristics of any elements or molecules.

However, fitting the spectral data still ruled out several atmospheric models, especially those similar to Earth. Their model suggests that even if the planet has an atmosphere, its atmosphere can only be similar to Mars/cloudy Venus/foggy Titan. Such an atmosphere is obviously not habitable.

The transmission spectrum data of the star LHS 475b obtained by Webb (black dots, the black lines indicate the error range) and the best fit of the spectrum data to different atmospheric models: the orange line represents the solar atmosphere model with the metallic abundance multiplied by 1000 times, the purple line represents the atmosphere model with water vapor, the black line represents the atmosphere model with carbon dioxide, the light pink line represents the atmosphere model with methane, the cyan line represents the Earth's atmosphere model, and the gray dotted line represents the model with or without atmosphere.

(Image source: Lustig-Yaeger & Fu, et al. 2023, arXiv:2301.04191v1)

So, if someone insists that LHS 475b is a habitable planet, it can only give a triple denial.

In fact, this observation by Webb only further confirmed the existence of a "terrestrial planet" (a type of rocky planet) similar in size to the Earth, and used the transmission spectrum obtained to constrain its atmospheric type.

The LF team's paper does not say at all that they have found a habitable planet (because it is not!). They emphasize the constraints on the atmospheric composition of LHS 475b obtained by Webb data and the sensitivity of Webb's observations - Webb is sensitive enough to brightness changes of less than 50 parts per million.

This is not the first time astronomers have obtained transmission spectra of rocky planets, as the Hubble Space Telescope and the Spitzer Space Telescope have previously obtained transmission spectra of some rocky planets and used these spectra to rule out some atmospheric models.

4. If you had to choose the "next Earth", which planet discovered so far has the most potential?

Having said so much, it seems that all the news is disappointing. However, we don’t need to be pessimistic: although LHS 475b is not a habitable planet, astronomers have previously confirmed some potentially habitable planets. For example, four of the seven planets in TRAPPIST-1 (TRAPPIST-1d, e, f and g) may be in the habitable zone, among which TRAPPIST-1d and TRAPPIST-1e have an overall average temperature of about 286.2K (13.1 degrees Celsius) and 249.7K (−23.5 degrees Celsius), respectively, and their masses are about 0.4 and 0.7 times the mass of the Earth, respectively.

A collection of artist's impressions of some of the confirmed possible habitable planets. The number in parentheses below each planet indicates its distance in light-years (ly). For comparison purposes, the sizes of Earth, Mars, Jupiter, and Neptune are also scaled on the right.

(Photo credit: PHL @ UPR Arecibo)

Among these habitable planets, the closest to us is Proxima b, which orbits Proxima Centauri .

Proxima Centauri is also a red dwarf star with a very low brightness, and its distance from us is only 4.2 light years. Proxima b is 0.05 astronomical units away from Proxima Centauri, and its overall average temperature is about 234K (-39 degrees Celsius).

An artist's impression of Proxima b (right) and its size compared to Earth (left). (Image credit: PHL @ UPR Arecibo, NASA EPIC Team)

For science fiction fans, Proxima Centauri is a well-known star. The "Three-Body People" in the novel "The Three-Body Problem" are assumed to live on a planet orbiting Proxima Centauri. Although astronomers did not confirm the existence of this planet (Proxima b) until 2016, many novels have previously assumed that there are planets around Proxima Centauri. In 2019, astronomers confirmed two more planets around Proxima Centauri, Proxima c and Proxima d. However, the existence of Proxima c was questioned in 2022.

Proxima Centauri and the nearby binary star Alpha Centauri form a triple star system. The novel "The Three-Body Problem" assumes that the planets of Proxima Centauri are unstable in the triple star system, so there is an alternation between the "Stationary Era" and the "Chaotic Era".

Will such a scenario really happen? In fact, the distance between Alpha Centauri and Proxima Centauri is far enough that it will not interfere with the entire Proxima system. The "era of chaos" imagined in "The Three-Body Problem" will not appear on Proxima b.

As things stand, if one day our descendants want to migrate to a habitable planet outside the solar system by spaceship, Proxima b should be the first choice. In other words, if we want to find a habitable planet outside the solar system, we have to ask the imaginary "Three Body People" for territory.

An artist's impression of the topography of Proxima b. Also shown is Proxima Centauri (the disk) seen above and the more distant binary star Alpha Centauri (the two dots to the upper right of Proxima Centauri).

(Image credit: ESO/M. Kornmesser)

In the novel, the technology level of the "Trisolarans" can crush the people on Earth; but in reality, there may not be any advanced life there, and the life on Proxima b may not be as advanced as the tardigrades. If there are alien life forms with a civilization far superior to that of Earth, they can easily block the sunlight with a coronagraph and take images of the Earth without waiting for Ye Wenjie to send them the coordinates of the Earth before taking action.

Therefore, the author believes that if we want to go to a habitable planet outside the solar system in the future, we can safely go to Proxima b to open up new land and rebuild a home.

Of course, these are just fantasies about the distant future. Although Proxima Centauri is relatively close, with the current level of technology, it will take tens of thousands of years for a spacecraft launched by humans to reach there; during this period, humans will have to reproduce thousands of generations on the spacecraft. In the foreseeable future, humans have no way to achieve such a flight. Therefore, even if Proxima b is indeed habitable, it will be difficult for us to reach it in the short term. Because of this, we must cherish and protect this precious and lovely blue planet - it is our only habitable home at present. [Note 2]

Notes

[Note 1] If a planet has an atmosphere, then the starlight that passes through the planet's atmosphere must be different from the unobstructed starlight. By comparing the difference between the two, we can get the transmission spectrum. Further study of the transmission spectrum may help us determine the types of elements or molecules contained in the planet's atmosphere.

[Note 2] Proxima b and other similar planets are potentially habitable planets, but not necessarily habitable planets. The Earth is currently the only confirmed habitable planet.